HAL,
Accurate figures for Sea King serviceability trends would presumably be available via a Freedom of Information request.
Although Sea King serviceability is far from perfect, the problems are to a large degree offset by having 2 aircraft at each base - so it's still very rare that neither aircraft is able to respond, and even now, after 30 years of service, both are frequently available at once (as in the Morpeth floods, attended by both Boulmer Sea Kings only 12 days ago). I would strongly suspect that having one new aircraft instead of 2 old ones would not improve availability of the 1st Standby aircraft - we've already seen the problems experienced by the S-92 in the last couple of months (and this isn't a dig at CHC/MCA/anyone else - just a statement of fact).
There has been a dip in availability this year while the new SKIOS engineers bed in - the problems have apparently been worst at Valley, due it seems to a lack of personnel. The same problem beset BRAMA when Hawk engineering was first contractorised, but the situation improved as time went on, and one hopes the same will occur with the Sea King contract.
TOTD