Cash in lieu should be the estimated cost of repairs, less your excess, less the VAT (until you can provide proof that you paid VAT on the repairs).
This is normally used in cases where the policyholder has showed an interest in retaining the vehicle and repairing it themselves.
However, this takes into account that you may use your own repairer (rather than the insurance companies approved repairer), and so may get a lowere labour rate, and/or use second hand parts to repair your car - basically to stop you making a profit from your accident (which goes against the principles of insurance - ie putting you back in the same position that you were in prior to your loss).
In regards to disputing the valuation, you have done the correct thing. The adverts should show vehicles of the same model which are comparable - ie they have the same or higher mileage, are the same age or older, and are the same model. By doing this, you can show that an older vehicle with more miles is worth more, so their value is unfair. However, if you can find adverts to show that their offer is fair and you;re just looking to get more, then be warned that they can find exactly the same adverts to support their value by showing that you could replace your vehicle for the same money, or possibly less. I have seen people submit adverts for F reg fiesta's in mint condition for �2000+, when their car is held together with rust and adverts for same condition vehicles show the cars for under �200.