I've been a bit dilatory - but here's the final (for the moment) version:

Stitch by careful stitch and line by line
The knitter sees the woollen sock emerge
And line by line and patient word by word
The sonnet links its syllables and rhymes.
Needles you need, (not two or three, but four)
Each one in careful tension with the rest;
Likewise your mind must gently hold the text
In place, and balance meaning alongside form.
Like a sock the poem must turn. Just so:
The hardest part of any knitter’s task.
The heel is done; you race towards the toe
And sew the final threads. So we just ask
Indulgence as we see the poem through
And hope our sonnet ends with neatness too.

So, with some advice from a friend, I've gone for this version. I'm still wondering about swaping the odd word. Perhaps 'patient' for'pondered'?
Though I like the assonance of pondered and word....

Stitch by careful stitch and line by line
The knitter sees the woollen sock emerge
And line by line and pondered word by word
The sonnet links its syllables and rhymes.
Needles you need, (not two or three, but four)
Each one in careful tension with the rest;
Likewise your mind must gently hold the text
In place, and balance meaning alongside form.
Like a sock the poem must turn. Just so:
The hardest part of any knitter’s task.
The heel is done; you race towards the toe
And sew the final threads. So we just ask
Indulgence as we see the poem through
And hope our sonnet ends with neatness too.

Except I've discovered some other jottings I made which lead to a different opening.

Stitch by careful stitch and line by line
The knitter sees the woollen sock emerge
And line by line and pondered word by word
The sonnet links its syllables and rhymes.

Put side by side, I'm not sure which I prefer. This might be an occasion for a second opinion.

Getting from here:

Line by line and stitch by stitch
The woollen sock is knitted
And line by line and word by word
The sonnet moves towards conclusion

to here:

Stitch by stitch and line by line the sock
Is made, colours resolved, patterns entwined;
And word by word and line by patient line,
The poet frees up the tumblers of the lock.

has been tortuous.  It's one of the few sonnets where I've really been held up by the rhyme.  So it was a bit like unpicking a lock.  LInes and stitches have been moved about so many times I dont know where they should be. But now I can see the whole thing and take a breath.

I've been working on the last six lines. I've moved much around, cut out bits and just left the parts that worked. The structure does make you economical with words!

Like a sock the poem must turn. Just so:

The hardest part of any knitter’s task.

The heel is done; you race towards the toe

And sew the final threads. So we just ask

Indulgence as we see the poem through

And hope our sonnet ends with neatness too.

This quatrian took a long time to come anywhere near satisfactory. Moving 'balance' to line 4 and putting tension in line 2 was the first stage. Then finding a way of rhyming 1 and 4 occupied me - nearly threw out 'form' several times but it is the word I want. So here goes:

Needles you need, (not two or three, but four)

Each one in careful tension with the rest

Likewise your mind must gently hold the text

in place And balance meaning alongside form

I’m looking at two lines now, where I think the form will quite easily reflect the meaning and where there is already a natural rhyme, or near-rhyme:

 

Each kept in careful balance with the rest

Likewise your mind must gently hold the text

in place And measure meaning alongside form

 

It pushes rest and text to the end of the lines and ‘in place’ drops to the next line, where it might be OK.  Not sure about ‘gently’ but we can come back to that

Line by line and stitch by stitch

The woollen sock is knitted

And line by line and word by word

The sonnet moves towards conclusion

Four needles hold the textile tube

Each kept in balance with the rest

Your mind must hold the text in place

And measure meaning alongside form

 

And like the sock the poem must have a turn

The hardest part of any knitter’s task

But once the turn is made you race towards the toe

And sew the final threads together

So, as we move into the final lines

We hope the poem ends with neatness too.

 

I decided that I would start writing with the previous ideas in mind and try to see the poem in terms of lines, regardless of whether they had the right number of syllables and certainly not worrying about a rhyme. By the time I got to the ninth and tenth lines, some were forming themselves into pentameter.  Now I feel I have a good structure in place with which to fiddle.

Apparently (I am reliably informed by at least two knitters) one of the hardest things to knit is a sock and the hardest part of the sock is the turn at the heel. This reminded me of the sonnet, with its 'turn' at the start of the sestet. It doesn't always occur, of course, or if it does, not there exactly. It set me thinking of the similarities between a sonnet and a sock.

This may take some time.

Like a sock, a sonnet should have a turn / a sonnet can survive without a turn / but a sock without a bend at the heel is hardly a sock at all.
A sock is held on four needles, all of equal importance. A sonnet balances its meaning with its structure but look at it how you will, a sock is clearly a sock, a sonnet just as clearly a sonnet. 
The knitter creates line after line and to unravel one, all the succeeding ones must go. In a sonnet you can tinker anywhere.
The sock is carefully woven, stitch by stitch, word by word.
If insufficient attention is paid, the thing won't fit!