Part-Ex is just the same as selling it to WBAC😄 someone's still gunna buy it
Just cut your losses and get rid of it, quickly
If you want to give it a bit of longevity and piece of mind for yourself, drop the sump and clean the strainer, then sell it
But I would get rid of it ASAP, all 1.0s are going to bomb in value soon anyway, once word gets around how unreliable they are and how much the belt change at 10 years costs
Take the £4,400 and run