Well, looks like my gamble has come around to bite me on the arse, one of the HDDs is reporting read/write errors, doesn't spin up properly and seems to be at death's door, so that's £100 down the drain as I can't RMA it
I need to get a replacement too and the cheapest are still around £140 (if you can find stock!)
The only good thing to come out of it is that I have more material for my unRAID guide to show what to do when your parity drive hoses itself...
I've switched the array offline until I can do further diagnosis on the drive or find a replacement. Thankfully its served its purpose, touchwood no data loss unless the other one craps itself - which it won't if I leave the array offline - anyone got recommendations on the best 3Tb model to go for?