Ouch!!! A gearbox failure is a definite shake of the head and a very deep dig into the pockets!
Having had a few more quotes, it appears the quote of £420 was too good to be true, they'd just quoted for the water pump! However, just received a quote of £763 inc vat supply/fit from a reputable local VW specialist, looks like I'll be booking it in there. There is a temptation to just ignore it, as from the good old VW technician iPhone video you get whilst they're doing the service, it appears to be a very minor damp area around the unit and I've never had to fill the coolant in the past year. Still, now it's been officially highlighted as a 'leak' I'd kick myself if ever it completely gave way and if ever I sell my beloved GTi, it'd make the car more appealing to any potential buyer.