Your question is hard to follow.
If I read it correctly, now that we're into the 3rd three-month period of the year (July to September), you'll be working 17 days in this month. Assuming that you work 5 days per week, Monday to Friday, you'll be finishing work on 25th July, which is the 206th day of the year. So your total holiday entitlement for this year (from 1st January to 25 July) will be 206/365 x 28 days = 15.8 days. (Many employers would round that up to 16 days but some might call it 15 days and simply pay you for the odd 0.8 of a day).
That 16 day entitlement (if that's what your employer rounds it to) includes all 'enforced holidays' (such as Bank Holidays, if you didn't work on those days) that you've already had, plus any other paid days off you've take already this year. What's left is what you're now entitled to. (The fact that your employer normally works to a pattern of 7 days off every 3 months is irrelevant. It's your TOTAL entitlement for the period orked that matters).
I've assumd that the 28 days holiday your employer gives you is the statutory minimum (for someone who works 5 or 6 days per week), including public holidays. If you actually get 28 days PLUS public holidays, please post again to explain the situation (also confirming the date upon which you'll finish employment).