horticultural grit

I'm not sure just the addition of grit is going to help there
Land drains perhaps.

 

Drainage channels the way to go!
I too live on the London clay belt, loads of organic matter and a couple of tons of pea shingle from wickes or whoever won't hurt!

 

Mine too, but 30 minutes after the rain the standing water has gone, except rarely when it has been very wet for weeks ...

If you have standing water for more than 30 minutes after rain stops, and in times when it has not been raining for really prolonged periods (like this summer), then you need land drains. Nothing else will work IME