I was right on the money with the rocker covers causing the burn smell
Make sure they do the full rocker cover service as per subaru recommended servicing... Subaru supplies a pack with all the parts and also the spark plug seals which get changed as part of this.
Just make sure they source the oem pack from subaru parts. Im assuming that's an AHG car yard, if it is they usually source the right stuff, if not ahg then I'd mention it to them so they don't take you for a ride.(I'd mention it to ahg too anyway, look like you know your stuff and they won't try bullshit you!)
edit: btw dealer saying that rockers always leak: no they don't always leak, only after years of driving when the gaskets start to wear away and not seal properly. The dealer should do this service for you now on stat warranty else it will cost you ~600+ later.