Your limiting factor is that requirement for a manual - when you get to a certain 'level' of car, especially diesels, they all seem to be autos or dual clutch's
Given you've had an e9x before, how about trying something different and going for that A5 3.0TDI you suggested. This is not a bad shout, it's different to what you've had before and I guess if you go for S-Line, you can't go wrong on spec as long as you have the optional Nav. Oh, they're also definitely available as a manual as a quick search of PH classifieds has just proven and it's probably one of the most powerful manual diesels outside of a 330d.
Damn, I forgot your monthly payment on the Zed was just £180....thats crazy low so you'll definitely end up spending a lot more! If this really is a cost saving exercise to get something more economical for work, are you sure you'll actually be better off given the much higher monthly payment you will make and the loss on the Zed? I know, it a really annoying question as you're trying to justify a change to yourself but you may as well just admit it...you're fed up of at the creaky Zed and want something else
You're unlikely to get those roof issues if you got a cabriolet A5!