Firstly is your garage integrated into your house or is it a seperate building? I'm guessing it's integrated due to the existing socket outlet? I'm also guessing this socket outlet in the garage is an unfused spur. The regulations state that you can only unfused-spur once for every socket outlet in the house. Fused spurs are unlimited.
Also an RCD isn't a protective device, it is merely additional protection for detecting small earth faults. It won't detect over current such as short circuits and overloads, therefore it isn't a fused spur and is not to be used as a protective device here.
Leave that existing socket outlet alone unless you want to replace it with a fused spur then you can run your four new sockets from it in a radial spur arrangement. RCD socket outlets would have been a good idea here but not neccessary.
What I'd do is connect your new sockets to your household consumers unit in a ring final circuit arrangement using 2.5mm cable protected by its own 32A MCB. You can do this slighly cheaper in a radial circuit using the same 2.5mm cable and a 20A MCB if the floorspace is less that 50 sq.m. Unless you can extend your downstairs ring final circuit to incorporate the garage sockets in one larger circuit? Just be careful you don't exceed the maximum 100 sq.m. floor space, and don't connect a 'ring within a ring'!
Don't wire your lights in a ring final circuit, they need to be radial only. A cable calculation should be applied here but in reality use 1.0mm if no loft insulation is present or 1.5mm if loft insulation present on the cable run. Same thing, connect them to your consumers unit on their own 6A MCB.
Just as a point of note, a ring main is used to connect sub-station transformers and is usualy 11kV, houses have ring final circuits. Also UK nominal voltage has been 230V ac since January 1st 1995.
If your garage is a seperate building then it needs a sub main SWA cable feed to its own consumers unit, buried at least 600mm deep. Also I'd use RCBO's instead of MCB's unless you can use your RCD to protect the new consumers unit (I'm assuming its a 30mA RCD?). A cable calc is needed here also to determine the conductor size as well as the protective device needed at your household consumers unit, among other things such as volt drop, maximum demand and diversity. That'll keep you within the 17th edition regs.
Between you and me this is notifiable work under part P of the building regulations and as such the buildings inspector needs to know what you are doing and he will need to see an Electrical Installations Certificate complete with a Schedule of Inspections and a Schedule of Test Results showing all your dead and live test results to demonstrate that you are a competent person. I hope you know what you are doing mate otherwise your buildings insurance could be void!
If you would like to find a reliable, insured and vetted tradesman in your area why not click through to the Find a Tradesman area of DIY Doctor: http://www.diydoctor.org.uk/find_tradesmen/ , complete the form and receive up to 5 FREE quotes
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!