Can anyone suggest a late flowering lavender?

Well most of them will be, John - they usually start flowering around May, have a bit of a pause, start again around mid to end June, have a pause and start again end of summer/early autumn.It has to be said, though, that I live in London, and it may be that this isn't true in more northerly, colder regions of the UK. The pauses in between give you a chance to clip the bush over, particularly after the first flush and the last.

Many thanks Bamboo - although I am in N Midlands, this is for my son's garden in Haywards Heath. I will follow your advice

I'd avoid L. stoechas, though, the French type lavender - they tend towards prima donna behaviour and don't always perform well.

Again, many thanks for this added advice - my future daughter in law desperately wants the garden to have lavender in bloom on their special day. Keep your fingers crossed for us

Well I will, but with our very unpredictable weather, who knows;-)

Maybe 'Goodwin Creek' hybrid, with excellent drainage. It has been known to flower into June, here in the desert. Feathery-leaved species are another possibility, but they are somewhat frost tender.