My friend normaly works two shifts of 16 hours per week but sometimes covers for others when they are off. Does she accrue holiday for these extra shifts as well?
No, not normally.
Holiday pay is based on one's contractual hours - and hence the proportion of those contractual hours compared to what hours a full-timer does.
There's nothing to stop a company doing something more than the statutory minimum, but many employers don't.
This issue is a constant source of gripe for part-timers who do lots of extra hours.
Normally in a situation like this, a 12 or 16 week average would be taken of her hours up to the point where she takes time off on holiday. This should account for fluctuations.
She should check her contract details for this. If she wasn't issued with a written contract, then I think the company is legally bound to provide her with one within 8 weeks of her requesting it.
It is not a simple yes or no. There are examples of employers giving for example a 15hr a week contract but regularly asking for additonal hours to be worked. The idea being, if the employer needed to cut hours then they would remove the additional hours without the need to go through the redundancy procedure (10% reduction leads to redundancy of the post). Also its cheaper if they do have to make the post redundant because it is based on contractual hours. However, if you are asked to do additional hours irregularly then holiday pay would not be payable.