Ken - it'd be well worth you planting a few plants around the edges of the pond to provide frogs with a bit of shade and somewhere to hide. and possibly get some small waterplants (if you want something edible - put some watercress it - it doesn't need still water - grows really well in my pond and keeps the algae away).
Harriet - try asking friends, neighbours, colleagues, etc. Never take spawn from the wild. I dug my pond last year, a bit late for breeding season, but this year have a load of toad spawn already. I'm very proud! (whereabouts in London are you? I live near London)
