Ian,
In the area you were in - if I'm guessing correctly - there was probably some WOODLAND. The chart is very accurate as to shapes; try and find a unique shaped piece of woodland, and look for it on the chart. The Forestry Commission does have a habit of cutting trees down, so this isn't foolproof, but it can be useful in rural areas.
The Welsh Hills...OK, any particularly high ones? This can be a clue, if you don't climb so high that you can't tell.
Use everything. The charts are extremely accurate, so even if you can find a river crossing a railway near a village of a particular shape, that might well be the only one like it in that area.
Don't despair. It IS difficult...but gets somewhat easier with practice.