There's two aux belts... The main aux belt that powers your alternator etc that is held in place with the tensioner. The other belt is only for your air conditioning & doesn't have a tensioner.
The recommended aux belt replacement (Both belts) is every 5 years/54,000 miles (Whichever comes first). You had your aux belt & tensioner replaced in 2015 & another belt replaced in 2018. Which belt was replaced in 2018, presumably just the a/c one? Normally, the tensioner is only replaced as part of the timing belt service (10 years or 108k). However, I think its best to change the tensioner too with each set of aux belts (Two belts) every 5 years/54,000 miles for peace of mind, especially knowing its a weak point on these engines. The tensioner itself is a £60 part give or take, so replacing that every five years in addition to both belts isn't too much more of an expense.
Bring it to the garage & get the squeaking noise checked out as soon as possible... If one of these belts fail, it can cause serious engine damage that could cost a fortune in repairs. If the garage say the noise is coming from the aux belt or a/c belt, then get both replaced with a new tensioner for peace of mind... That would be my suggestion.
