Sorry to jump in on the discussion, but I have flown Pumas and I am now flying the Merlin. Not in the RAF, but in the PAF - Portuguese Air Force.
I operated Pumas for 4 years in 711 Squadron in the Azores islands, and 2 weeks before we completed the phase out, we actually received a group of engineers and pilots from the RAF (both pilots were Squadron Commanders I think), to survery our a/c, and we were glad to see that everyone from the RAF liked the look and feel of our Pumas. Portugal ordered the first production aircraft from Sud-Aviation in the late 60's, but we upgraded them during their operational life, and we didn't use them as much as you do. I believe normal total time for our airframes were around the 6000/7000 hrs (in 38 years of service). In the early 90's we upgraded the engines to the Makila 1A1, and we gained a lot of available power (the MGB was not upgraded, so MTOW remained 7400Kg). Your pilots told us that in Iraq or Afghanistan that extra-power would be most welcome. I have to say that I never saw NR droop while flying our Puma, not even taking off at high AUW or landing at 7000 ft, which we did sometimes.
Then we bought the Merlin, and it is a great aircraft. That is, when it is outside the hangar, because as soon as it enters the hangar, they don't want to get out. There are no spares, period. And this is hurting a lot, we are having a lot of problems qualifying new pilots because there are no available hours. A lot of this problem is our fault, but the Canadians have the same issues with spares.
The Merlin brought an improved capability to us, doubling our SAR coverage from 200 NM to almost 400 NM, but curiously, we don't have a single heliport certified for 15600 Kg in mainland Portugal, which means that we stopped doing EMS service for the civilians, which we sometimes did in the Puma. It is a very good a/c, but the lack of spares and cost of flight hours has us now wandering why we didn't buy the Cougar...
I don't have knowledge about the specific situation of the British Armed Forces to give an opinion about what you should buy, but just thought you'd like to know what our experience has been.
Best wishes
Pinho