So the van's been in the dealership for a starting issue recently, and none of the technicians could work out the cause of the blinking. Didn't realise before but the driver's window wasn't opening and closing properly and the lock/unlock buttons weren't working either. One of them suggested it was probably some aftermarket wiring job which could be very expensive to track down and sort out, AND wouldn't be covered under warranty.
Luckily I found a video online which had the solution. The wiring which goes to the window switch has the same plug on it as the one going to the door handle and locking buttons. Someone has put them on the wrong way round even though it's pretty obvious which one should go where. Swap them round and all sorted.
The trickiest bit for me was working out how to get the door panel off! Couldn't find instructions anywhere until I found the video below. There are several screws to undo, several 7mm sockets and one T20 torx. The ones behind the reflector and the door handle latch are the hardest to find.
https://www.youtube.com/watch?v=NCKemI9Lj4E -- solve the flashing lights/ dodgy window issue
https://www.youtube.com/watch?v=MdG71h4Zn5w -- get the door panel off. Mine's a LWB and there's a document holder on the bottom of the door - unlock it and three 7mm screws are visible inside.