I do realise it's not as simple as 8*37.5*52 because of the way holiday pay works. But how do you work it out?

It ought to be as simple as that as an annual salary doesn't vary for holiday periods. It should be an annual figure divided into pay periods. With this assumption the figures you quote make it £15600 pa

I always multiply by 2000. It overstates it a bit, but it’s not a bad estimate and the maths is easy to do in your head! Similarly, if I have an annual salary then dividing by 2000 to get the hourly equivalent is nice and easy.

it also depends whether there's opportunity for / expectation of overtime, whether there's any enhanced rates, e.g for weekend work, any bonus scheme. and what they mean by full time - can be 35 or 40 or more hours a week

37.5 hours. Office based role so I'm assuming no overtime. That may be wrong but it's better to assume not.