What rear derailleur do I need? I could really could do with a hand here, I'm currently restoring a old Raleigh Milk Race bike which has friction tube shifters fitted. I want to upgrade the rear derailleur but I'm not sure if I have to buy a certain type. It has a SunRace one fitted, and the front one is a Shimano one.

The bike is a 7 speed.
Almost any rear derailleur will do.  You aren't limited by the shifters (since they're friction shifters, they have no index incompatabilities) and it's a 7 speed, so anything that is rated for 6, 7, or 8 speed will work fine.  Go to a local shop that sells used parts or a local community group and take a look at what they have in their parts bin.