Nothing, apparently - Neil gave it a full 12k/2 year service, which was due anyway, and it fired right up. He set the mixture, changed the fuel filter, plugs etc, but found nothing specifically wrong. I reckon it was just sulking about sharing the garage with the R1!
Doesn't matter - I did the intelligent thing and searched for an original post - glad it's ok now.
I haven't had my own SP3 long and it seems to be running ok but not as strong (I don't think) as my SP4. It doesn't need servicing so I'll just "watch" it.