Hello David,
Welcome to the Forum.
So sorry to learn of your plight.
After only eight weeks of ownership, and buying from a garage, you will have Consumer Rights.
What warranty do you have ? Even the shortest warranty should be 90 days; more than 8 weeks.
I would be speaking with Trading Standard, Citizens Advice, and maybe a Solicitor, before speaking with the Garage again.
They may take responsibility, they may not; you will need to give them an opportunity to rectify the issue before taking further action.
Sending you a re-conditioned torque convertor is, imho, pathetic. Was the warranty parts only ?
Please send me a Personal Message and let me know the 'garage' you bought from.
At this stage, better not to name (and shame) them in this public forum.