Hi Chris.
The thing to look out for is the tall (almost full boiler height) tank in the back left corner of the boiler as you're looking at it.
The old design of Micro Accumulator Vessel tank is basically two plastic tank parts bolted together. They have replaced this with a one part stainless steel tank.
I have some photos of the old tank if anyone's interested.
I called them and booked a service out on their fixed repair fee of £250. Responded very quickly, sent one engineer out to investigate and confirm fault, and then sent a two man team out the following day to actually do the repair.
Drained the boiler, took the unit off the wall, swapped the tank, refitted, all in just over an hour.
I then rang up glowworm's support desk and stated that I was a bit unhappy about the fact that the tank had split in the middle, and if it wasn't a design fault, why would they change it to a one piece tank. Two minutes on hold and they came back with an instant refund.
Can't blame them really for trying their best to get out of it, all I'll say is a big thanks to Louise below for giving this information in the first instance. Thank goodness for the internet.
Good luck.
Rich D
August 2010