Unfortunately hellfire you're not the first, nor I very much doubt the last, to fall at the mercy of the ISP on this one.
This is a very common problem that many ISP's use (petty in my opinion), used to be a big thing a couple of years ago but not so much these days.
Best suggestion would be to try and speak to Sky, nicely sometimes works, but try calling after 11 at night, you should get hold of a UK based operator.
@ rob: the mac code that hellfire was after is the MAC - Migration Authorisation Code. This is the unique identifier for your broadband service and is used to transfer your service from one ISP to another. Only definitely works if you are using a BT line though (Sky's service retains the BT line, so you should opt for the BT Option 1 package and online billing for the cheapest line rental and Sky's call package sits on top). It's a bit like porting your number from one mobile/landline operator to another.
If you are on a full LLU service, like most* Tiscali/Carphone Warehouse and many others, you should be aware that you are giving up your BT line. If you want to return to BT in the future you'll likely have to pay a connection fee!! Currently about £120 :o
Also with full LLU services, MAC's are 1) not guaranteed to work or be accepted by the receiving ISP or 2) may not even be given!!! Although this is a breach of the code of practice Ofcom introduced last year, which basically states that you should receive a MAC within 5 days of requesting it, although if a termination has been actioned then this may not be possible. The MAC is only valid for 30 days, after which you'll need to request a new one - but be careful, some ISP's consider a request for your MAC to mean termination and may well switch off your connection even if you don't use the MAC!!!!
Oh, the mac that you mention rob is the mac address, the unique code for each ethernet device - this is a globally unique address (but is easy to spoof). My mac address on the pc i'm using to type this, is different to the mac on the router and on your pc. This is not the same as an IP address and is not to be confused with the Migration Authorisation Code, that can only come from your ISP.
Got loads to do and I'm sat here, see you later, hope this all helps.
h