Discrepancy between offer letter and contract. Which takes precedence?

My offer letter, issued in August 2016 and signed by me shortly afterwards, states:

"The post is for 18.75 hours per week (plus weekend work), your usual days of work will be every Monday and Tuesday 8.45am to 5.15pm with half an hour lunch break and alternate Fridays 11.45am to 5.15pm. On occasions there may be a requirement to vary the working pattern to meet operational needs. Your days and times of work will be agreed with your Line Manager."

I started work in September 2016, to this pattern, but was not included on the weekend rota and have not worked at weekends.

In October, I received a contract (or "Statement of Written Particulars in accordance with Employment Rights Act 1996"). It states:

"Your normal working week is one of 18.75 hours. Working patterns will be determined by your line manager." "You will not normally be expected to work outside your contracted hours."

I have just been issued with a weekend rota for the period April to September. Of the 27 weekends, I am rostered to work 15 of them.

Am I right in thinking that there's a discrepancy between the offer letter and the contract? And, if so, which takes precedence? Am I obliged to work weekends? I have a disabled son, an elderly mother who lives with us and a partner who definitely is contractually obliged to work at weekends, so I'd prefer to do all of my hours (plus voluntary weekday overtime, which I'm often needed for) during the week.

Thanks,
all_greek_to_me