The mech is bent so needs replacing. According to the Specs its a Shimano SLX Shadow Plus, 10-speed, SGS cage.
Will this be a like for like replacement ?
https://www.merlincycles.com/shimano-m670-slx-10-speed-rear-derailleur-58856.html?utm_campaign=googlebase-GB&utm_source=googlebase&utm_medium=shopping&utm_term=MTB+Rear+Derailleurs&ucpo=12682&gclid=Cj0KCQjwqvvLBRDIARIsAMYuvBFq4D0RRWIzbw1pmfXhl972Tm5sOL-i46yoO7X---f0Yr07dmpN99AaAsnzEALw_wcB
I can't find the M- number on my Rear Mech but the one on the link is an M670, does this matter when replacing mechs ?
That mech would work (as would any 10spd Shimano MTB mech) but not it's not like for like. That's a normal shadow mech not a shadow plus. Shadow plus is the clutch mechanism. You'd need this one I think (out of stock unfortunately) https://www.merlincycles.com/shimano-slx-m675-10-speed-shadow-rear-derailleur-60109.html
If you want to buy from Merlin then this [url= https://www.merlincycles.com/shimano-deore-m615-shadow-rear-derailleur-62709.html ]deore[/url] version
or this [url= https://www.merlincycles.com/shimano-xt-m786-10-speed-shadow-rear-derailleur-58057.html ]xt[/url] version have the clutch.
On your old mech the m number is probably on the back of the main body of the derailleur. I don't think you will be able to see easily if the wheel is on the bike.
the latest SLX (7000 series) also has a 10-speed version, and is clutched.
CRC has them in SGS in the clearance.
http://www.chainreactioncycles.com/shimano-slx-m7000-10-speed-rear-derailleur/rp-prod148232