Thanks Donnie - I thought I would look at a Behr unit. No way could I do the work myself, so I'll supply the part to the tech and risk having to redo the labour if the part fails. Your reply also suggests that the aftermarket units are slightly different to the MB part, such that the connections will be more fiddly - is this what you mean?

Do you have an opinion on the point about having to replace most of the bits when converting from R12 to 134a because "the glue connecting the pipes together melts"?