Have a leak on the gearbox and sourced it to a seal, getting harder to go through gears and mechanic allows its due to low oil from the leak but last year I had a faulty slave cylinder and replaced it.
Do yous think it could be the problem more than the leak?

I would top up the gearbox oil pretty quick if its suspected to be low , damage from low oil can't be undone and will eventually result in a replacement box , I wouldn't of thought the slave cylinder would of failed so quick but there is a possibility