On an imperial gas meter 1 unit is about 32 kWhs
On a metric gas meter 1 unit is about 11.2 kWhs
On an electricity meter 1 unit is 1 kWh

Convert your meter readings to kWhs as above (those are approximate numbers for gas - exact sum is on your bill.)

For gas, your 2680 kWh is per annum. In a quarter, that will be 2680 / 4 or 670 kWh. So the first 670 kWh cost 7.145p the rest cost 3.396p. 7.145p - 3.396p = 3.749p. So that's an extra 3.749p for the first 670 kWhs each quarter. 670 x 3.749p is £25.12 per quarter or £8.37 per month.

So the cost of your gas is ( 8748.93 x 3.396p ) + £25.12 or £297.11 + £25.12 or £322

But to answer your actual question: as it is a 94 day quarter, the number of Tier 1 units will be a bit more. (2680/365) per day times 94 days or (2680/365) x 94 = 690. So for this bill, about 690 kWhs will be charged at the higher rate.