This question gets asked a lot. The most frequent cause is a lack of pollination. I think I read something (or saw a program,) about Europe also having trouble with robust Honey Bee populations. You also mentioned a 'really wet summer'... pollen can become sticky/clumpy and be prone to viability problems under wet conditions. Cool temperatures can also cause problems with pollen production.

Secondly, if the vine already has a number of fruits growing, it will abort what it cannot support - pest problems can exacerbate this problem. And, rather than switching to flowering/fruiting/seed production, too much N-based fertilizer can keep the plant in a vegetative growth phase, causing fruits to be a secondary effort.

And lastly, I would not discount your wet weather as providing conditions which host fungi. Squash and pumpkins can be prone to many fungi/mildew issues. These usually affect leaves first, but there are fungi that do attack fruits.