have a CH boiler on my landing - a Potterton Flamingo 20 -30. I also have
a carbon monoxide alarm across the landing from it and just recently the
alarm has gone off a couple of times.

Turn it off immediately. Whilst it may be a false alarm, you should always
take a CO alarm seriously.

I know the boiler is an old one but, just off the top of your head, could
it be anything simple to fix?

We can't possibly know from the description. You should get someone out to
look at it, really.

Christian.