Thanks again everyone for your support, it really does help to know that others know how I feel....
Over the last few days Felix seems to be a lot better, he is still falling over if he's going around a corner too quickly, but if he concentrates and walks slowly he can manage walking without any tumbling.
We've been back to the vets this morning and she agrees that in himself he's a lot better. She's given us something else alongside the Metacam (name escapes me, begins with an S!) and she wants a urine sample as she's not totally convinced that the arthritis is his only problem. So now I have to try convince him to pee in a litter tray
She wants to see him again in 3 months, fingers crossed (again) that he'll continue to improve...
Wishing you all a very merry Christmas, and thank you again for your support
PS - I have photos on my phone of the big black monster but don't know how to upload them?! Am a technophobe...!