I wont bore you with all the technicalities but there are wildlife laws concerning the sale and taking of amphibians, including eggs.
Great crested newts and Natterjack toads are fully protected by law. It is illegal to even shine a torch into the pond as it disturbs them.
Common toad and frog are only really protected from sales. However, it is illegal to take them without permission from the landowner and it isn't recommended anyway as there is a big risk of spreading diseases such as red leg.
Amphibians are incredibly mobile and forage over a large range and will search out new ponds. It is not unknown for them to travel a couple of miles from their orginal breeding pond (GCN breeding ponds are protected by a surrounding 500m of habitat because of this).
Amphibians basically form a metapopulation which is a group of areas of suitable ponds, foraging habitat and hibernacula where the breeding population in individual ponds will vary from year to year depending on conditions. If a population is wiped out for some reason it will usually be recolonised by the neighbouring ponds in the metapopulation so if they aren't present one year they will likely be there next year, at least to check it out. Because of this, if there are any suitable ponds in your area in nearby gardens etc (or even hibernating amphibians) they will colonise the pond naturally within a few years.
One thing to remember is amphibians and fish in ponds don't mix. Fish will hammer frogspawn and newt eggs and tadpoles so you will end up with a very low population of them but lots of toads as toads and their growth phases are slightly poisonous and generally not eaten