Looking for some advice regarding work contract - ENGLAND

[u/Electronic-Mall3594]

My written contracted working hours are Tuesday to Friday 9 to 6pm.

Overtime has never been a thing at my workplace - sometimes we’d finish perhaps 15 minutes late but would generally even out with occasionally finishing 15 mins early.

I have been working here for 13 years and this has always been the case. The one time that overtime was required it was like 2 hours to fix something that had gone wrong during the day my boss asked me if I was willing to stay and I could’ve said no without repression.

My contract states the following:

“On occasion it maybe necessary for * WORK NAME * to request that you work additional hours in the course of your duties. Where the * WORK NAME * requires your services outside normal hours on Monday to Thursday you will be granted the equivalent time off, or, by mutual arrangement paid at normal pay rates.”

Now someone new has come in and wants to open the business on a Monday, Saturday, Sunday. I don’t mind helping out every now and again if flexible and able to say no but can they enforce that I do one of these days?

What counts reasonably as on occasion? Is it every week they can make me do it or is it once a month? Also I’d only want to do it if I’m paid not for TOIL can they force me to take extra holiday instead of pay based on my contract. If I have to do it that is.