Hi Plotman.Thanks,Teresa for your vote of confidence in me! making my 2 ponds into one would be wonderful,but as one is raised and one below ground,with a few feet in between,it would be a nightmare to arrange.
However,I did redo my nature pond to make it larger.I removed all the plants from both in and around the pond,and kept a small amount of the mud at the bottom,plus I kept some of the cleaner water.then I put the rest through a sieve to catch all the insect life.The frogs and newts that I found at the bottom I lost count of!! But yes,March is the best time,i would say.Then we took out the liner and dug wider and deeper.i did have a bog area,but I used that to make more pond space.it worked very well.We put a better liner in and now the extra depth means more frogs stay in there over the winter rather than invade my fish pond!You need at least 18 inches,more if poss.For a fish pond,2 ft is the minimum.
Plotman,is it to be a nature pond or a fish pond,and are you lining it? You will need to make sure the ground is firm where you reline.For a fish pond you need to have a good quality underlay.Nature ponds you can get away with sand.
Give me more details on what you plan to do,if i can be of help i am only too glad.