Hi Steve,
Yes I drained it myself, but only the bare minimum. When I use the car regularly it turns over just fine, always starts first time, it's only if it's left for a short
while (week or two) it turns over a bit slower, but still starts first time. I will get a new battery anyway, think 13 odd years is long enough in service.
Before I went away on Monday I had another look at it, took the tank cap off, put the heater control on max and fired it up, when it got hot the level
rose to the point it overflowed a bit, rev it up and the level would go down, squeezed the top hose a few times and some more would come out, repeated
this a few times, put the cap back on then went for a blast down the bypass, got back home and checked the level which was fine, so went away on our jollies,
did around 100 miles made up of motorways and towns before the 'low coolant fluid' came on, checked level when cold next day and it was still fine, used it all week,
came home and it has not come again at all, so maybe, just maybe the air lock has worked it's way out, we'll see, many thanks for all your input and if it raises its
ugly little head again I'll let you know.