Thanks for your advice. Jack is around 11 years old (he was said to be about 9 when I had him, had him two years) so it's an estimate. He's healthy and has a fine appetite, he's bright in himself and is fine it all areas, eating, toileting etc.
I think the small weight loss must be down to him doing a lot more excercise with his new toys, nothing suggests to me that there is anything wrong with him and I'm not worried about him, as he hasn't given me any cause to worry.

It just occured to me that perhaps I could worm him. I think I'll leave that for now as I don't want to cause anything myself.
If he loses anymore weight then I will definately take him to the PDSA to have him checked out, I think I'm so used to seeing him being a fat couch potato that the shapely Jack was a bit of strange occurance, lol