It's about 25 yrs old. For the last 2 yrs the pilot light keeps going out (sometimes after hours, sometimes after weeks), at times it lights first time no problems, othertimes it can take ages and on two occasions i've had to walk away to save me from ripping it from the wall...... Any clue as to whats happening or is it a new one?
I had a similar problem. In my case it was traced to abnormally high levels or direction of wind from outside the house entering the flue and extinguishing the pilot. The remedy, which was never 100% successful, was to fit a windbreak in the form of a metal shield in front of, but being careful not to block, the flue terminal. Keeping the door to the small room in which the boiler was situated also helped.
I have this boiler. Mine uses a Honeywell Q309A thermocouple, which you can get from Plumb Centre etc. I don't find they last more than a year or two but they are easy to replace. Mine just stopped working yesterday as it happens, but it isn't the thermocouple this time... Engineer coming tomorrow.
This time it was the Gas Control Valve. Another Honeywell part, but still available and replaceable, but only by a CORGI registered Gas Engineer and at a price. His comments were that the boiler was top of the range for its time, which is probably why there are still a few about. The 'RS' part of the name stands for Room Sealed and it's important that the front panel of the boiler itself is not cracked and its small glass panel at (the thermocouple entry) is also intact, otherwise the combustion products can enter the room, which can be lethal. If they are, he advises that those parts are no longer available and it will be time for a new boiler, as the engineer will condemn it, if the casing is split.
If your boiler is still OK, do get it serviced to keep it clean, which will help to avoid heat building up in the wrong place, which can cause the damage mentioned.
Finally, being old, it will be fairly inefficient and burn more gas than you need to, so it may be an idea to start looking at potential replacements.