A few tweaks half way through - and then we're done:

There’s been some clouds; there’s been a little sun,
We’re optimistic that a barbecue
Is just the thing. Look up – there’s still some blue
Above the trees where swifts are having fun
Scooping up a feast of evening flies.
Unwrap beef- and veggie-burgers – we’ve tons
Of sausages. Perhaps we need more buns
And beer as well – let’s not economise!
The coals are lit once more. This time they’ll take –
And we can have a break to slake our thirst.
Then, as the bangers hit the grill, the first
Raindrops plop down. Then more. What is it makes
Us do this every year? Do we have no sense?
Ah, the triumph of hope over experience…

I'll be back in September. 

It was not so much rain stopped play but visitors.  Now the sonnet is finished but I'll try to remember how the process went.

I had the idea of finishing with 'The triumph of hope over experience' and thought 'sense' as in 'why don't we have more sense?' would make a good rhyme. 

The effort of getting the coals to light came next to  mind:

The coals are lit once more. This time they'll take
And we can have a break to slake our thirst...

Extra internal rhyme, no charge.

Now for the rain:

Then as the bangers hit the grill, the first
Raindrops plop down.

Plops seemed exactly right. That was how it was.

So - the final six lines go like this:

The coals are lit once more. This time they'll take
And we can have a break to slake our thirst...
Then as the bangers hit the grill, the first
Raindrops plop down. Then more. What is it makes
Us do this? Do we have no sense?
The triumph of hope over experience!

The whole thing needs polishing but the I can see the final sonnet emerging.

Unwrap the beef and veggie-burgers. We've tons
Of chipolatas but we need more buns
And someone needs to make a salad
And get some beer,

OK hold on.  This needs a bit of organising.

There's been some clouds, there's been a little sun
We're optimistic that a barbecue
Is just the thing.  There's still some blue
Above the trees...where swifts are having fun
Snapping up the evening flies
Unwrap the beef and veggie-burgers. We've tons
Of chipolatas but we need more buns
...and something to rhyme with flies

I know what's going to happen in the sestet:- rain. 

Enough to get going....

There's been some clouds, there's been a little sun
We're optimistic that a barbecue
Is just the thing.  There's still some blue
Above the trees...

What was above the trees?  Well, in our neck of the woods it's swifts so I'm going to transfer them to Harborne with a bit of poetic licence.

...where swifts are having fun
Snapping up the evening flies / insects?

A feast of evening flies.

Scooping?   Swooping?

Unwrap the vegieburgers and...

There's been some clouds, there's been a little sun
We're optimistic that a barbecue
Is just the thing - more relaxed /  casual than eating in
With place mats, cutlery and all that stuff.

The coals are finally lit once more. This time they'll take...

This is how I ended the previous blog:

Gathering thoughts for the July sonnet.  It's not the facts of the matter which are so crucial, though they are essential, it's the approach, the slant - a media person would say, the angle, I suppose.  I've now written a lot of celebratory sonnets for people having anniversaries, birthdays, weddings etc and realise that I do need lots of snippets about them - but the next stage is the hardest: deciding what the angle will be. What will the poem hinge on? (Angles, hinges... I'll be talking about opening doors onto a new vision soon.) So also with a July sonnet about a rainy barbecue. What's my underlying (or over-riding) feeling?  Probably the way we do these things in spite of the elements - and manage to wring enjoyment out of it.  We'll see...

And now, thoughts and notes and jottings are taking shape - - -

Blow on the grey chunks of charcoal
Don't breathe in
Send out for firelighters
Have a brolly standing by
There's swifts screeching overhead
Snapping up insects
While we unwrap veggieburgers, bangers
And somene makes a salad
rushes out to buy some buns

Just disjointed memories so far - but that's one place to start.