There is a minimum right to paid holiday, but your employer may offer more than this. The main things you should know about holiday rights are that:
* you are entitled to a minimum of 5.6 weeks paid annual leave (28 days for someone working five days a week)
* part-time workers are entitled to the same level of holiday pro rata (so 5.6 times your usual working week, eg 22.4 days for someone working four days a week)
* you start building up holiday as soon as you start work
* your employer can control when you take your holiday
* you get paid your normal pay for your holiday
* when you finish a job, you get paid for any holiday you have not taken
* bank and public holidays can be included in your minimum entitlement
* you continue to be entitled to your holiday leave throughout your ordinary and additional maternity leave and paternity and adoption leave
My interpretation of that would be she could expect to be paid 5.6 weeks holiday at 31 hours.
There was provision to calculate holiday based on the average hours worked over the 12 weeks previous to the holiday pay being paid, but I'm not sure if that was from the Working Time Regs, or the "standard" construction industry Ts&Cs.