B.J wrote:I've since realised that the wrong oil filter has been fitted

. I'll change that first and let you know if it solves the issue. Is it expecting too much for a so called vintage specialist garage to fit the correct parts or am I being being naive?
Oil filters don't really differ much inside - it's more a case of "does it fit"? And, clearly, that one did. But given that the engine didn't start life in that van, I'm not sure you can complain at 'em too hard.
Another vote here for low oil level - classic symptom, in any car. If the level's fine, then it's probably a wire loose and shorting out as it swings around.
ObG's theory's an interesting one, too - although gut feel says "But it's never done it on roundabouts before", it's not impossible. Is it the first time you've had the van in LHD-land? If so, then it could be that it's just something you're not seeing in the UK, due to the slosh-slosh-slosh of an enthusiastic roundabout being the other way round to normal.