Getting copies of adverts etc can't hurt, but contrary to Internet lore the insurers don't have to actually pay much attention to them. From the actual ombudsman website:
'To decide whether a valuation is fair, we compare it with prices in online motor trade guides such as Parkers, Glass, CAP and Cazana.
We’re likely to agree with your valuation if the amount is in line with the guides. But if we think your valuation is unfair, we’ll tell you to adjust it using the guide prices.
If a guide price is significantly higher or lower than the others, we may think it’s reasonable to ignore it. This depends on the value of the vehicle. For example, a difference of £200 would be significant for a £1,000 car, but not for a car worth £9,000.
Customers sometimes say the amount they’ve been paid is unfair because they’ve seen similar vehicles advertised at higher prices.
We wouldn’t normally place much weight on adverts to decide whether a valuation is fair. Differences in mileage or year of registration can significantly affect value, and in sometimes the vehicle actually ends up selling for a lower price than advertised.
Although, you should be aware that more recently, we’ve been told by some trade guides that generally cars are selling at or close to advertised prices.
Adverts may be helpful if the complaint involves a classic or rare model. Or if they strongly indicate that the guides could be wrong.
If your customer bought their car second-hand just before the claim, we’d still expect you to have used the guides to get a valuation.
The guides should give an accurate valuation and it may just be that the customer paid more than the guides suggested it was worth.
Although, if there’s a big difference you should think very carefully about whether this suggests the guides are wrong. In this case, we’d expect you to review other evidence, such as adverts, to get to a fair valuation.'