Annoyingly, the back of my iPhone (glass?) screen has cracked after I dropped it on the bloomin' floor today

Is there a way to get this repaired (after which I think I'll need to invest in a case)? Roughly how much does this cost?

Apple will do it for £25 I believe, seems a bit steep to me but then I'd just do it myself. You might be able to find one of those random bric-a-brac shops that does cheap phone repairs, but I have no experience of them so I would be careful.

Is that via a mobile phone shop such as Carphone Warehouse or do I have to send it away?

They should do it in an Apple shop there and then for you. You might need an appointment though. Carphone Warehouse would have to send it away, so I wouldn't go through them.

if this is an iphone 4 you can get the buy the back covers from ebay and all you need to do is remove the 2 screws on the bottom and the back comes off then replace with new one and fit screws back, walla

If it's a newer model though it will have the pentalobe screws, which means also buying a new screwdriver. Also, not everyone is comfortable taking apart their phones, especially as it voids the warranty.