Take the money and go to CEX to buy a used 3090ti fe from there for £910 or a 3090 for £700 and you will get a 2 year warranty with it as you clearly need a Nvidia card with 24GB and a 4090 is as you said £500 more.You entitled to think whatever you like, but after their first offer I am allowed to be sceptical of any other offers they provide me. I not looking for a free upgrade as you imply, I would have loved for them to just send me an identical 3090ti back to me, and avoided all of this.
Call it a lesson learned, I'm in same boat as you I have 2 x 3090 in Nvlink for my work and purchased a 3rd one as a spare for this reason as I need to have dual card setup and they need to be the same make and model for the Nvlink bridge to fit as all 3090/ti's are not the same where they place the fingers for the Nvlink.