So is the argument buy new Chinook vs. marinise Merlin Mk3? What are the differences in cost there? Is the £0.5 billion to simply marinise the Mk3, or does that include an upgrade similar to MCSP and growth capacity?
Marinisation - not required.
I believe that Chinook would still have to be marinised if it wants a floating home. Marinisation is not just about treating the airframe and folding blades. I'm sure that Merlin and presume that Chinook has an option to manually fold. The equipment fit required for nautical operation (or at least littoral operation) has to be considered. EMC is a massive consideration with the numbers of high power transmitters and highly sensitive recievers on board, what is the current EMC clearances for the Chinook? Merlin equipment is designed to work near maritime radars. What are the current ship handling operating limits for the Chinook? Merlin was designed to be operated at sea.
Deck space - not an issue
Operationally it is necessary to occasionally "lilypad" what is the smallest deck the Chinook can do this to/from if the flat top needs to be cleared? Or if you are involved in large scale SAR.
There are merits to both solutions, to be honest the answer is not simple, if it was the matter would already be decided. I just wanted to express an opinion on some of the negative things that have been said about Merlin and skipped over for Chinook.