@andrewhesketh
What is the part number shown on the belt?
Asking because looking at this [https://www.4hotpoint.co.uk/washing-machine/wmfug-742p-uk-f086248/belt/catalogue.pl?path=52516:595447&model_ref=11459948&refine=belt|supplier] of the belt, there are 4 belts available for your model and they're not all exactly the same specifications.
The part number gives the specs i.e.
1201 J6 PJE = 1201mm length J section 6 ribs PJE elasticity value
1194 J5 PJE = 1194mm length J section 5 ribs PJE elasticity value
1208 J5 PJE = 1208mm length J section 5 ribs PJE elasticity value
1194 J6 PJE = 1194mm length J section 6 ribs PJE elasticity value
Whilst 2 are the same length they have a different number of ribs and the other two are different again.
I'm sure that you would have noticed if there were a different number of ribs on the new pulley and you did say that it's an effort to reattach the belt but maybe the belt is worn or has stretched a bit too much.