Had another email from HMV today (they have a bad reputation for online orders of games - looks like they've gone into overdrive with this to try and repair it!!
They confirmed that it was dispatched yesterday, and gave me a Royal Mail Tracking Number - Royal Mail doesn't recognise it specifically yet, but it does seem to identify it as "Next Day Gtd by 9am Special Delivery"
Price for this service (at retail) starts at over £10 - HMV cannot be making much on GTA
(they're giving away 1000 XBox live points too)
Didn't arrive today - think it's unlikely to arrive tomorrow (Saturday) but there must be a darn good chance I'll have it Monday