Longer

Hi.
Just took the scoob for a blast on the motorway for the first time in a while. (I live on the Isle of wight)
I wasn't driving it too hard but then the engine management light has come on and going into 5th gear is clunking. It will go into other gears ok with no clunks although 1st and reverse don't always go in first time. Sometimes you need to double clutch.
I'm wondering if anyone can help with a couple of things. Firstly, I've been told the gearbox issue could be a sync issue and will probably be best to get a new box. Is this more likely or could it be linkages or selector arm? I haven't got a clue.
Secondly I'm looking to get diagnostics done. So ideally I'm looking for someone to plug it in and diagnose the fault code and confirm the gearbox issue at the same time. I'm in South London until Wednesday. If anyone can recommend someone in that area it would be spot on. Thanks in advance.