I have two 1210s, a mk 2 and a mk 5. When I let go of a record on my mk 2 it seems to take longer to catch speed compared to my mk 5, and feels 'slack'. Any ideas what this is?

You said one of your 1210's seems to be lacking tourqe? There are several posibilities. And none of them involve replacing a "belt". All of the SL1200/1210 series. Turntables are direct drive and do not have a belt. The motor's drive shaft runs right up through the center of the platter. No belt. Rather than going over all the possible causes and solutions, I am posting a YouTube link that spends about ten minutes on the subject. I am also including a separate link to a site that sells the original oil for the SL1200/1210's.
My 20 year old MK2's seemed noticeably stronger after cleaning and recoiling the drive spindle. Good luck. EW.

is your 1210 1200 lacking torque

KAB Electro Acoustics http www kabusa com

I’m happy to help further over the phone at https://www.6ya.com/expert/erik_149c4c1c216e42f2

Being the older model, the chances are that the drive belt on the Mk2 turntable has got worn and shiny with use. It may also have stretched a bit. If you are using the tables for 'scratching', the fact that you are holding onto the turntable when the motor is running is placing a strain on the belt that it is not really designed for. I would recommend changing the belt - you can easily find spare belts for sale on the internet