I keep having to repressurise with the valve below the boiler, but it just drops again. I assume this means something somewhere is leaking, but how do I find out where? We had our bathroom renovated a few months ago and the radiator was taken off and put back again. On one of the pipes there is now some rust colour, so think there has been a small escape of water - would this be enough to cause it?
We had the same problem and the plumber who installed radiators had to come and fix it. I would write down what the pressure was every day because it would give an indication of how big the leak is. It might be a valve in the boiler. Our plumber asked us to go with a piece of toilet paper and check each valve in radiators. Any wetness would indicate a leak but in your case streaks speak fir themselves. I would contact the plumber.
We had the same thing with ours, we'd wake up to find it was on 0.0 bar, top it up to 1 bar, but then when it was on the pressure went up to 3bar instead of the usual 1.5. Turns out we had a leaking expansion vessel in the boiler. If the pressure is going up high when on, it's probably something to do with the boiler and does need to be fixed as high pressure in a boiler isn't a good thing. I'm not sure what it's called, but there's normally a small curved pipe just which runs from the boiler outside and when the pressure is too high, water will leave via this, so look around outside and see if you have water underneath.
However, if you are topping up the pressure to around 1bar and it's going up to about 1.5bar when the heating is on and then dropping slowly then I suspect you could well have a leak. Have a good look around where all your pipes are, look at ceilings where pipes run above.
We have a flexible hose in the boiler which gets clogged periodically. DH sometimes waggles the hose, which is enough to get things moving. We have had to replace it once though, but it wasn't an expensive job.