Just thought I should give an update. I had been experiencing some (sometimes severe) hesitation problems. I've replaced the DME relay, but hadn't had the chance to give it a proper drive since my battery failed shortly after (Murphy's law?).

Got a new battery now, and went for a "spirited" drive yesterday. After approximately 15 mins of driving, all the hesitations were gone! I love this car!!

I should note that I had hesitations over the entire rev range, not just around 4k (although the effect got worse at higher revs)

Thx guys!