Are you better replacing these in pairs or is singly fine? Got an advisory for my near side front on my mot and just wondering if it's worth doing both!

Single is fine, Could fail years apart.

I've always replaced as needed. Remember that the nearside possibly has more stress due to sunken drains etc I'd check on whether or not it's a ball joint or drop link for the front ARB. I've just had a MOT fail for "ARB ball joint, nearside front  excessive wear " and "ball joint offside front , cover damaged ". i've put that down to the MOT PC format.

( If I'm preaching to converted, please excuse),  but if you're only going to replace worn ball joints, then a usefull tool is a wedge ball joint splitter. Remove nut, insert tool and bang and the taper is broken. I'd suggest if you have to split joints ,you'll want to re use then get one of the screw types . Check length of threaded section on new one against the old,.if the same, back the lock nut off one turn ,BEFORE SPLITTING. Use a pipe wrench to hold steering rod and remove old joint, counting number of turns. Replace new one ,and check that lock nut only needs a turn to lock. ( sets tracking approximately as before). Get tracking checked. IF one of the monkeys tells you that the other ball joint needs moving, and it is solid, UNDER NO circumstances let him use heat.