Finally, I got around to The Great Bunging!
Thanks to all the help here, everything went very well. I had both rad. valves off and despite a constant small leak which I soaked up with towels, there was no problem at all. I initially had a hose on the doc and let that do its thing. Once it had stopped I prepared the rad. and valves and dropped it back into place.
As I mentioned earlier, the boiler is an old Potterton Flamingo that 'doesn't always re-light.' Draining it down and then creating another problem trying to persuade the old b****r to light up was not something I really wanted to do, it takes so much time anyway!
So I just took my time and about 2 hours later it was finished, the boiler was running and I was a happy chappie.
Thanks again for all your help and advice, bunging is something I will certainly try again,
TerryH