In reply to Dominicandave:
Bought my Mum a cheap one on eBay last year for her birthday, somewhere around £60 - ideal for messing around on the beach, but wouldn't put much faith on durability.
Can't find the exact model we got on there now, but this was the one I was looking at originally - the price has come right down when I looked: item #272220480288
Otherwise, look at a 2nd hand Sevylor, or something, on eBay - they are much tougher, more durable and easier to paddle as they can be pumped up harder and the seats are better.
Decathlon usually have a few on display if you need some ideas.