The following is from the Manheim website. It seems your compaint should be to the vendor, Ford Dunmow, as Manheims claim to be just an intermediary to the deal between Vendor and Purchaser.
1/ A Vendor warrants to Manheim that it will immediately inform Manheim if a vehicle is known by it not to be Roadworthy.
2/ Offer is Accepted by the Vendor a Contract of Sale is formed between the Purchaser and the Vendor.
Manheim is not a party to the Contract of Sale and has no liability or responsibility in respect of the Contract of Sale.
3/ The fall of the hammer will mean that Manheim has accepted an Offer on behalf of a Vendor, but Manheim may also accept an Offer verbally or in writing on behalf of a Vendor and an Offer shall be deemed as Accepted whenever an auction or sale of Goods ends and Manheim has received (on behalf of the Purchaser) the Acceptance from a Vendor.
4/ Check if the vehicle is ‘Assured’ which means it’s had a 15 point mechanical check from a Manheim Assured Inspector. These vehicles carry a compulsory premium which gives a 3 day warranty on the mechanical report.