I've just purchased a towel rail radiator to replace an old bathroom
radiator. The problem i have is the towel radiator is only 35cm wide
whereas the old radiator was 45cm. Also the valves screw in vertically
(from the bottom) and the current valves screw in horizontally. The
pipes to all the radiators in the house are 10mm and are behind the
wall (plasterboard) rather than under the floor boards.

Is there an easy way (suitable for a novice plumber) to replace the
valves and reduce the width between the valves from 45cm to 35cm
without draining and disabling the central heating tank in the loft? Or
could someone point me in the right direction to an idiots guide on
this?


There is no idiot's guide, you will have to do the job properly man.
This means a complete drain down and a modification to the existing
pipework.

If I were you I would mount the new radiator so that one of the CH
pipes (which I assume comes up from the floor) feeds directly into one
of the new vertical valves. This will mean discarding the existing 90
degree valve and probably cutting off a few mm of pipe in order to
accept the new olive.

Then you will have to fashion an "S" shape out of copper pipe (two 90
degree elbows and a couple of small lengths) to allow the other CH pipe
to connect to the other valve, obviously discarding the other existing
horizontal valve.

You will need a length of (probably 15mm) copper tubing, a blowlamp,
flux, wire wool, solder, elbows, and a pipe cutter.

After the job is done you will have to rebalance the system for the new
rad (see FAQ).

Note that the wall fixings for these towel radiators require very
precise positioning of the pipes! (No margin for error!)

Luke