Any tips for the best way of checking for rust in the rear inner arches, without any dismantling? My B is sound (I've had the rear bulkhead out), but I'm going to look at another one tonight, as a possible present for my daughter. The seller claims it's rust-free, but the MOT history from 2011 says it's been welded (the first time I've seen welding mentioned on an MOT advisory list). Unhelpfully, it doesn't say where.

That's strange Gareth, you would have thought it had to have a fail ticket first stating it's fail reason.
As you know the Barchetta is good at hiding it's rust issues. The inner sills are impossible to detect without removing the floor mat, unless they are really bad. Rear arches perhaps you could give them a gentle press,listen for any slight movement perhaps,

On inspection last night, there was some rust around the inside of the rear arches, and a very small amount of bubbling spreading under the paint on the horizontal seam, close to where it meets the arches. Not dissimilar to mine, in fact. Other than that, it seemed pretty solid. Impossible to tell if there's worse going on underneath. It's been dry-stored by the current owner for 10 years, and used very little in that time (1k per year of summer jaunts on average), but I've no idea how it was treated for the first 12 years of its life, and the service history is extremely patchy. It's also apparently had its engine rebuilt at some point, due to a broken cambelt. Seems to run OK now. Bodywork isn't perfect - the yellow paint is faded (it's a visibly deeper yellow on the part of the hood lid that's covered by the hood when it's raised) and there are some minor dents and dings. Ho hum.