dozy:
My pay slip ( agency ) just says , holiday amount = 125.21 ( paid weekly ) no mention of rate / hours etc
That looks like 12.07% on top of your gross pay.
As you say you like it in your pocket rather than theirs. There is one benefit of it building up and being paid out when you actually take holiday is that you have weekly “tax free” amount of £190 for National Insurance and if you get paid nothing for any particular week, then you lose that “National Insurance free” allowance.
(although for you earning over £967 per week your marginal rate for National Insurance is only 3.25% - so that’s all you’re losing out on).