In what way, i had a run from the house of around 20 foot and for the 4 years i used all the powers i have from a 400w hand drill to a mig welder/plasma cutter with no problems.
I did ask about the set up with a sparky and he said it was all fine but no need for the house end kill switch i used at night to cut the power.
If instead of that "Kill switch" you fitted on of
these
then it would be closer to what is required
Any way i have come to the idea of just using the spur to run the cony and then a new feed from the main box to the new workshop, so that mean the longest run from the cony spur will be about 7m long, i plan to have 3 maybe 4 twin sockets in the there and 4 down lighters.
If you run any cables then you need to test even the best electrician can make a mistake something like
this 
is really the minimum test requirement to ensure no silly mistakes. Really they are not good enough and a full test set should be used but I am a realest and I know the DIY guy will not buy test gear costing £750. As to consumer unit since the RCD FCU is going to have a 13A fuse unless the breakers are less than 13A then rather pointless. A second switched FCU with 3A fuse for lights is more order of the day.
However you have a maximum of 13A or 3000W continuous and less than 95A for 0.4 seconds i.e. max earth loop impedance for a 13A fuse is 2.42 ohms. A mig welder/plasma cutter is likely to use far more than this will provide and likely it would blow the 13A fuse quite regular.
The standard ring circuit will supply 30/32A total depending is fuse or MCB and standard 2.5mm cable is rated at 20A approx so it relies on there being two cables feeding every socket. Spurs are allowed but only to feed a single socket as then the 13A fuse in the plug gives the protection required however the spur is also limited to 3 meters and has to be where damage to cable is unlikely. If multi sockets are required then you need to fuse at source with the fused connection unit. As the length of cable increases you then get volt drop and also the resistance in the cable can mean the fuse will not blow fast enough (ELI being too high) using a RCD will of course not help with volt drop but it will help to ensure if there is a fault it will auto disconnect.
As to legal aspect this is covered by Part P which in turn says you should follow BS7671 or similar to ensure it complies as although the BS7671:2008 is not law it can be used in a court of law to show negligence and so it may as well be law. Part P requires some work to be registered with the LABC and to do the job legal likely you will need a new circuit from the consumer unit. Since the LABC have a very high minimum charge over £100 often it is cheaper to get an electrician who is a member of a scheme to do the work as he can self certify so not have to pay the LABC before doing the work. He will also have the meters required to test the work and ensure it is safe.
As so well already put you can drive a car with no brakes using gears from A to B without having an accident many times. However you know that is not a responsible act and in the end something will go wrong and an accident will happen. It is same with what you propose.
I note the
Martindale plug in tester 
is slightly cheaper at around £43 and it does give some indication of earth loop impedance and same site shows some full 17th Edition (BS7671:2008) testers at £400 but by time you take the money for test equipment and the LABC charges and take that away from the labour costs for a registered electrician the whole idea of DIY becomes rather pointless.
