Hallo, I've nearly finished building a patio, and I've had a jumbo bag of builders' sand delivered to level it all off. However, I now see that the manufacturer of the patio slabs I want to buy from recommend a granular sub base, not sand. Would it be a good idea to lay a thick DPM on the sand, direct under the patio slabs in order to stop the sand retaining water in wet weather? (usually plenty of that in South Wales). Your expert advice gratefully accepted!