Hampstead
Last time I checked, and I got this from the CAA in writing via my AME, the JAA PPL/IR needs the Class 1 audiogram. The requirements are on the CAA website somewhere and IIRC there is a max 20db difference between the two ears.
Rather bizzare for a PPL/IR though I suppose relevant to an airliner cockpit where the people don't seem to wear headsets much, and if they do they sometimes wear them on one ear only. It would be very hard to have a conversation in a jet cockpit if you've got one slightly bad ear.
englishal
If you have a JAA PPL, flying a G-reg plane, and you are flying on business, and you are an employee (or a Director) of that business, then it is classed as a Private Flight, and not only it is OK to do it on a PPL but also the company can pay for the whole flight, i.e. no need to worry about the CAA PPL cost sharing rules.
This is frequently done; the plane is owned by a separate limited company which invoices yourself (if it is your own pleasure flight; you are then free to recover some of that cost under the PPL cost sharing rules if you had passengers), or it invoices your business (if it is a business flight), or it invoices somebody else (if that person/company rents the plane).
The above is very common practice under both G-reg (both Public CofA and, in certain narrower circumstances, Private CofA planes) and N-reg operation. The pilot does have to be an employee of the company on whose business the flight is being done (though he does not need to have any connection with the company that owns the plane, so the plane can e.g. be leased), and he must not be contractually required to fly the plane.
The above is my understanding from having looked at it many times. The CAA itself is reluctant to comment on anything like this; usually they don't reply.
I don't think N-reg operation differs from the above, except that any renting (and all three cases above do amount to renting) requires a document called a "dry lease".
My concern was that with a "piggyback FAA PPL" (which is what that pplir.org page describes) one might not be able to do the above. This is an entirely separate issue from the UK "PPL cost sharing" thing.
Personally I would go for a full FAA PPL and perhaps let the JAA one lapse; I believe one can fly a G-reg on an FAA PPL, VMC only or something like that.... presumably you lose any IMC Rating but then you've got the FAA IR with which you can get the IMCR just by filling in a form and writing a cheque... but presumably if you wanted that one day you would have to get the JAA PPL back. Oh well... of course it all depends on what access you have to an N-reg plane. For an owner, the choice is trivial.