why do I get blossom drop

Looks like a male flower. What variety is this? You need either a bona fide female to go with it or a self fertile variety such as 'Jenny' or 'Solo' that produce both male and female flowers on the same plant and will self set if you are wanting fruit.

I always though Kiwis needed a fairly long hot summer to set fruit - are you sure you're not too far north?

My one has never managed to set fruit either....well it set them - but they got nipped in the autumn frosts! I think we ARE too far north to get viable fruit - unless under protection. It makes such a wonderful climbing feature - my one winds itself up a massive eucalyptus and I have just let it and given up on the fruits!!

Well, in Northumberland, the Kiwi 'Solo' at work (outside) sets fruit. We are however not as far north as you.

I think given a sheltered site on a south wall you should still get away with it.

Yes - I guess a couple of hundred miles would make a bit of difference - and the it IS on a sheltered south wall.
Mind you I don't remember what the variety is - I planted it in about 1985 before I retired. I just enjoy it magnificent long tendrils and whopping leaves as it climbs!