Jonathan I tend to agree with David.(posted 7500 days ago)If the lender has supplied proof of marketing then they have evidence of attempts to mitigate their loss.
If Halifax have not supplied such items then you have an argument if they have supplied then the only other possibility is how long the property took to sell. If Halifax agreed to the first offer made and this was within a month or so of the property being up for sale you have a good point otherwise you may find a nasty judgement for £8,000 being registered.
It is a shame that you have not issued a SARN and checked their evidence I would ask for strict proof within the court process.
If you have something to loose from a judgement then really you should try and settle this at the 'court gates'
The fact that the two valuations are from agents associated with your lender does not hold any weight unless you have proof of corruption.
Good luck