Sorry but it does come under the control of the building regs as it is an outside application and as others have said it depends on several factors.
Just repairing something to reinstate it back to how it was doesn't make it safe.
When I moved in my shed was wired up from the house's lighting circuit to 2 double outlets and a strip light using 2.5mm twin and earth clipped to the fence. Now would you consider it safe to just replace that cable because it was there before.
Believe it or not sometimes the regs are actually there to protect you.
I know it is small job of replacing the cable but the red tape won't allow it. (i think)
that what I copied from the BAS link above....
DESCRIPTIONS OF WORK WHERE NO BUILDING NOTICE OR DEPOSIT OF FULL PLANS REQUIRED
1. Work consisting of—
(a) replacing any fixed electrical equipment which does not include the provision of—
(i) any new fixed cabling; or
(ii) a consumer unit;
(b) replacing a damaged cable for a single circuit only;
(c) re-fixing or replacing enclosures of existing installation components, where the circuit protective measures are unaffected;
(d) providing mechanical protection to an existing fixed installation, where the circuit protective measures and current carrying capacity of conductors are unaffected by the increased thermal insulation.
(e) installing or upgrading main or supplementary equipotential bonding;
(f) in heating or cooling systems—
(i) replacing control devices that utilise existing fixed control wiring or pneumatic pipes;
(ii) replacing a distribution system output device;
(iii) providing a valve or a pump;
(iv) providing a damper or a fan;
(g) in hot water service systems, providing a valve or a pump;
(h) replacing an external door (where the door together with its frame has not more than 50% of its internal face area glazed);
(i) in existing buildings other than dwellings, providing fixed internal lighting where no more than 100m2 of the floor area of the building is to be served by the lighting.
2. Work which—
(a) is not in a kitchen, or a special location,
(b) does not involve work on a special installation, and
(c) consists of—
(i) adding light fittings and switches to an existing circuit; or
(ii) adding socket outlets and fused spurs to an existing ring or radial circuit;
3. Work on—
(a) telephone wiring or extra-low voltage wiring for the purposes of communications, information technology, signalling, control and similar purposes, where the wiring is not in a special location;
(b) equipment associated with the wiring referred to in sub-paragraph (a).
(c) pre-fabricated equipment sets and associated flexible leads with integral plug and socket connections.
4. For the purposes of this Schedule—
“kitchen” means a room or part of a room which contains a sink and food preparation facilities;
“special installation” means an electric floor or ceiling heating system, an outdoor lighting or electric power installation, an electricity generator, or an extra-low voltage lighting system which is not a pre-assembled lighting set bearing the CE marking referred to in regulation 9 of the Electrical Equipment (Safety) Regulations 1994; and
“special location” means a location within the limits of the relevant zones specified for a bath, a shower, a swimming or paddling pool or a hot air sauna in the Wiring Regulations, sixteenth edition, published by the Institution of Electrical Engineers and the British Standards Institution as BS 7671: 2001 and incorporating amendments 1 and 2.
