wow 30 yrs is a good run, sounds to me like your diaphragm needs replacing in the diverter, you can buy a diverter repair kit for around 10-15 pound, pretty straight forward to replace if you know what your doing, would def look at replacing now though as your only throwing good money after bad...
So left in the bedroom it would be affected by the towel rail ? so lets move it to the landing where there is no heat source, Hmmmm clever, while the landing is cold the rest of the house where you spend all your time is bloody cooking :(
Thats the idea mate, if the rest of the radiators in the house have been sized correctly (which I doubt very much) then they should heat up at the same rate as the bedroom and once the bedroom is at the correct set temperature then the rest of the house should be too, unfortunately not many...
Vaillant 618 has a built in bypass so no external bypass required, plus building regs require trv's to be fitted in all living spaces, ie bedrooms and living rooms except where your room thermostat is located