MJ,
Going back to your statement regarding the 90 day rule. There is no requirement for the pilot who "needs the practice" to log anything other than the fact that they completed 3 take-offs and landings. Thus another PPL (who could be say the owner of the aircraft) can sit in the right seat and log the flight as P1. The only requirements are that a) The non-current pilot was the "sole manipulator of the controls" and that the "Owner" was not paid for sitting in the right seat.
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As for aerobatic instruction - Beagle may be barking up the wrong tree unless it is known that the FI providing the said aerobatic instructional flight has a UK licence. People who hold JAA licences from other countries do not have a "no aerobatic instruction" restriction. However, one would have to say that any FI worth their salt would not consider teaching an aerobatic course unless the had completed the required training. But a one-off trial lesson may be something different (provided that they can do safe aerobatics) subject to insurance limitations.
PPLs doing Club checkouts is very much dependent on the individual club set-up. Beagle agrees that in a multi-owner group, there is no problem.
I believe that the only problems arrise in a "Club" environment. "Club" being the profit making commercial enterprise version of a club found in the UK.
In the UK "Club" the aircraft are operated for profit. If pilot X is not current then they are unable to fly. Thus the club can not profit. If the CFI tells a PPL to fly with pilot X then the club is making a profit from that PPL's flying (even if it is as a passenger). That makes it public transport and illegal.
Another way to look at the checkout regardless of the "club" set-up is - Can one seriously make a club rule that a pilot is not competent to fly the aircraft if they have not flown during the previous 28 days - then turn round and send them flying with a passenger? Clearly, the two are not compatible.
Regards,
DFC