Derek, if there are frogs spawning in the pond in the field next door then I'm sure you already have plenty of frogs on the allotments - they spend most of their year in long damp undergrown and travel quite a way back to their spawning ponds. They're mainly nocturnal - I bet if you're on your allotment at sunrise in midsummer you'll see plenty - the allotment is probably also on the regular round of your local heron who will know exactly where to find frogs first thing in the morning 
If your pond dries out in the summer then it's unlikely to have the necessary plantlife and little invertebrates that provide tadpoles with the right food to reach adulthood. Also, if conditions are not right some tadpoles remain as tadpoles and do not metamorphose until the following year, so if the pond dried they would perish