Help from our resident electricians please.

We had a shower cubicle in the downstairs lobby, we have taken this out and aim to turn the lobby into a utility room,.
My question is, there is a twin and earth cable that fed the shower, this goes back to a 30a breaker in the fuse box. Can I fit a fused spur (with a 13a fuse) at the end of this, then continue the cabling to fit a double socket under the worktop or will I need to change the breaker.
The drawback with this is that the 30a is twice the width of a 15a.
Can anyone help.?

Change the breaker to 15amp & put in a blanking plate to cover the space left.

From a practicle point of view what you are suggesting will work, no problem. The cable between the fused spur and the socket could be 2.5 mm twin and earth, which is of course thinner than the cable from consumer unit and therefore easier to terminate in the socket.
You could change the 30A breaker for a 15A and put in a blank in plate, provided that it will supply any items you plug into the twin socket. A 3Kw washing machine will draw a max of 13A and a 3Kw drier the same. If both are on at the same time the draw could 26A, but the 15A breaker (MCB) will trip at about 30A so that's ok, how long the demand exists will affect the actual tripping current.

Good reading is at http://www.kevinboone.net/cableselection_web.html

The regulations, known as part P, allow you to replace an existing socket, but require a competent person to provide a new socket. If somebody considers himself competent then he can do the work. The trouble comes when the house burns down and the subsequent investigation says it's due to the new socket and the insurance company rub their hands and refuse to pay out.