getsometimein,
Your figure of £700 million a year running costs now appears to relate to MRA4, so, whether or not that figure is correct....
Once again, I would ask what savings have been achieved by folding the MR2 fleet early? That is a different question to any savings made by slowing introduction of the MRA4....
Regarding shortages in the RN Merlin fleet, is it aircrew or groundcrew (or both?)? I believe I have read on other pprune threads that some RAF groundcrew were/are(?) on loan to the RN rotary world. If you are talking about aircrew, how long would it take to turn a RAF mult engine jet pilot into a rotary pilot on RN exchange, 2 years maybe? The only Nimrod aircrew who could move across fairly quickly, if there is actually a need, and the RN training machine could cope with the burden, would be a few acoustically trained WSOps.