Electricity in porch

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What I find worrying is that the OP's builder is going to do the work, but they don't know that you can't have multiple sockets spurred off a ring and after consulting with an electrician its still not clear that they actually know what they are doing. Perhaps someone who knows the regs and has the skills to do a good job should be doing the work? 

As others have said, to get the result you want (multiple sockets added to this ring) the regs require one of :
a. the rig is opened and the sockets inserted into the ring, rather than being spurred off
b. a fused spur, and then you have have whatever arrangement of sockets you like downstream of the fuse, but bear in mind the load implication of that
c. to spur each new socket off at a different point on the ring. Unlikely to be practical in terms of chasing walls etc.

As the householder, the buck stops with you (the OP) for ensuring the work is safe and compliant. If you're going to have someone who isn't a qualified electrician do electrical work I think it is prudent to check that they know what they're talking about and that the solution they are proposing is compliant with the current regs.