Well, I went to my GP yesterday, she took a good look at me and told me that since my symptoms haven't really changed much over the last 18 months she's as certain as she can be that I don't have a brain tumour.
I've kept working with my CBT materials and strategies, which she encouraged. I have continued to sleep well and that makes a big difference to my ability to think logically and not catastrophise too much.
I think the next step is to spend less time on this forum since that is a bit obsessional in its own way, and to get on with life. Thanks to all who have posted their stories and listened with patience. Onwards and upwards.