How do I complain about Recommence Trade In service?

I want to know how to complain about your trade in partner service (with Recommerce) as they have not paid out the promised bonus payment and I feel that my trade in experience was falsely represented and deceitfully managed

I purchased a Surface 7 laptop via the MS website and followed the instructions to trade in my Surface pro 6 with Recommerce(TM). I sent them my Surface pro with the relevant documents (24/9/25). I received an acknowledgement of the receipt of my Surface Pro and a confirmation of my estimate saying they would check it was valid (£36 + £250 bonus) (3/10/25). I received an updated valuation (7/10/25) which I had to accept and then on (12/10/25) I had confirmation (congratulations!) of a payment of £277 (£27 + £250). However, on 13/10/25, only £27 was paid in to my bank account. I followed up with them on 13/10/25 and waited expecting a follow up payment (given the reference to 15 days for payment on the website) and then followed up again on 23/10/25 when I was asked to mail another inbox (one email I received was in French!) on 25/10/25. On 27/10/25 I received a reply saying that they had asked me for documents in a mail and that they could no longer pay my bonus payment. I have re scrutinised all my correspondence and I cannot find any such request. The fact that they confirmed the payment and within 24 hours did not honour the confirmation indicates that there is an error on their part. Together with the fact that I have not received any request for information. I have complained to them and I am registering a complaint with you as clearly this is an issue with your trade in partner. I only engaged in the trade in because of it's association with Microsoft. Please tell me how I can escalate this and get a response. I have been a Microsoft customer both personal and in my work in education for many years and I am horrified at this experience.