Thanks for those replies Potkettleblack and Froglob.
I understood that if a non JAR 25 aircraft such as the Twotter was required to fly two crew for the operation then these hours would count towards the multi crew time required for the issue of an ATPL. At least that was what was stated in the 2007 version of LASORS, which is in case you are not familiar with it, essentially the rules and regulations of the UK CAA for licencing. The 2008 version is less clear though.
I have a friend who has been recently flying a Be200 air ambulance in Scotland as co-pilot/safety pilot. He tells me although he can log the hours towards the ATPL for multi crew time, he needs to have a JAR 25 type rating in order to have the ATPL issued and apparently he cannot captain the Be200 under multi crew operation unless he has an Atpl.
Therefore, he is moving on to a JAR 25 jet within the company in order to get the ATPL and then will be able to move back onto the Be200 as Captain.
It appears to me that in order to fly an aircraft such as Twotter or Kingair as Captain you don't need an ATPL when it is a single pilot operation. However, if you were to fly Captain when it is a two crew operation then you need an ATPL.
What are your thoughts on that? Its all very confusing isn't it!