GAH!  Useless. 

I asked Scan to hold-off dispatching the order because I wanted to amend it (Case, PSU and Fans).  No problem they said - not due some parts until 16th anyway.

What did they do?

Dispatched it today...  Grrrr.

Will I have to pay for the return of the items?   

I doubt you will have to pay for the returns seeing as you told them to hold the order as you wanted to make some changes. Maybe they will even do a collection on delivery arrangement (that's what I would do if I was still in the game)