If you don't fancy fronting it out with Currys or SKY,you can buy an HD box yourself off the net.
I got mine for £199.
If you already have SKY+,it's a straight swap.
You still have to phone SKY and get the HD subscription though.
That only takes a few minutes.
BTW,they don't hassle you about where you got the box,not at all.
Any problems with the box,you don't even have to go to the seller.
Just phone SKY as normal.