The Wildlife Trusts suggest giving tadpoles cooked lettuce - steamed is ideal.

You can also buy daphnia from pet shops when the poles are a bit bigger.

If your pot has steep edge, Si, then even with a ladder it might be more suited to newts. You can buy 15 eggs from Charles Snell (he has a special license to sell them) for around £7. I think he also sells frog spawn so it might be worth Googling him if you can't find any from neighbours.