I thought that I could control the hot water independantly,
but it seems that I can switch it ON indepedantly, but if
I switch the central heating on then the hot water is on
anyway...

You may have gravity circulation. Could you describe the number and sizes of
the pipes coming from the boiler. You should have a gas supply pipe and two
pipes for the central heating. If you have two more pipes (possibly larger
than the others), you have gravity circulation. This is bad.

Christian.