The salvage yard are implying that there's £3k/£4k of repairs, so why is it a cat D?
If they're saying it's 'light damage' only, and is worth £18k, then why would it have been written off by the insurance company when they would only have done so if the repair cost was over 60% of the value of the car (i.e. £11k)?
Remember, 'light damage' to a salvage yard is not the same as 'light damage' to Joe Public. A scuff on a bumper is light damage to me, a car that has dents in every panel (e.g. in the case of vandalism, or low speed spin into a guard rail) but no structural damage would be light damage to a salvage yard.
Even if you bought it for £10k, repaired it yourself for £5k, it's still only worth about £12k as a repaired cat D.
I'd guess it needed 2 bumpers, 2 front wings, headlight, bonnet, radiator, headlight washers, etc. if it was to be brought back to 100%, but then it'd cost much more than just panel beating/straightening/filling.
Plus if the work has not been done by a professional, then I'd assume corners had been cut everywhere to keep costs down.