I have a brand new combi running 18 radiators + 1 shower in my house. No
complaints there.

Problem is that the radiators seem to be getting very cold very quickly as
soon as the boiler is switched off. My suspicion is that the cause could be
because some of the pipes (copper) hat feed the downstairs radiators are not
insulated. I have a 1m high space under the house, where the pipes are
located, and it is freezing cold and drafty there.

Does it make sense? Will it be a good idea to insulate the pipes? It
shouldn't be a very big job.

What shall I use? I was thinking about something like this (
http://www.screwfix.com/app/sfd/cat/...0943&ts=33331), but: 1. it
seem to come in various levels of thinkness (which is suitable?), and 2. it
may be tricky to fit in some places.

Any advice will be much appreciated.