Well, it seems that after alot of head scratching and even consulting several mechanics, none of whom were able to identify the problem, I eventually got to the bottom of the issue (for me, at least). Mine is a 2007 1.8S, by the way.
The surging/hesitancy seemed to be getting worse if anything. I had a few people attach diagnostics to the car, but there were no fault codes nor engine managment lights. Which I am grateful for!
I got myself a bluetooth OBD and the only thing i could see odd was the fuel trim being higher than you'd expect. It was displaying 20% rather than averaging zero, and I think that it's a fine-adjustment to the fuel being delivered.Now, I had to look up what this meant and a fair bit of on-line research, but t was pushing me in the direction of the pre-cat lambda sensor.
I then had a look around and got in touch with a supplier who was able to corroborate my findings and a replacement item was just over £170.
I fitted it myselk which was a bit awkward (two electrical connectors, one small retaining bracket and getting the plug out required several new swear-words), but the result has been very satisfying after driving around the problem for so long. It fuels and picks up just as you'd want it. I hope it stays like this, as the car has 110,000 miles on and is is great shape. I was offered £500 for it as a trade in last year and whilst £170 was a bit suck-it-and-see, it's great to have it back to good. I'd like to thinkn that the fuel economy might improve, but with the car's short-run lifestyle, it'll not show much.