Butyl liners are the best in my opinion.
you will have to get as many stones out as possible. when we put ours in we also put sand down and old carpet. then just to be 'safe' we used old wool blankets too.

just a thought a preformed fibreglass/liner would work but that brings its own problems such as making sure it is level and packing the soil in around the shapes. but less likely to puncture.

Carpet isn't extreme at all - its recommended for those circumstances. Better safe than sorry. And make sure you get a butyl liner not a cheaper one as they don't last well.

Thank you all again for your help. My garden is gradually taking shape (with all your good advice) and by the end of this year I am hoping to have all the terracing done and a path laid. The pond should be full and it will on to the planting. I have made the classic mistake of purchasing ever plant I like - not what I need - so when I need to sort out the plants I will be in a muddle.
Thanks again Cecelia

don't worry Cecelia. we all buy plants then think 'where will I put it?' I have a collection, awaiting planting, by the back door. they drive hubby mad !

Haha Cecelia - this is just why my garden is really somewhat of a hotchpotch. Some people plan a beautiful garden, some buy what they like when they are tempted and are either artistic enough to arrange them well or just enjoy them wherever they have managed to fit them in.

Yes Steragram that's me without the artistic bit.

and me too. I have planted out a few today as there is space in the ground but whether they go together only time will tell.