So that means that each women paid £9 for the TV as they got £1 back. 3 x 9 is 27 + the £2 the assistant stole
That's your mathematical error.
Here's two work-around to present the real answer:
Now that it costs 25 instead of 30, this means that at least one person paid more than the other two (if you wanna do fractions, be my guest). So, 9,9,7 satisfies this, as well as 10, 8, 7, etc. If we give back 3, that means we'll have 9,9,10 or 10, 8, 10, or any other combination. Then, it's easily seen that the two extra dollars kept will add the total of 30.
Second way:
Currently, each woman paid 10, to make 30. That means, when you have 5 going back to them, they will have paid 9 apiece with 2 extra. However, you do not add this 2, you subtract it since you're initially subtracting (10-->9) so 27 - 2 = 25.
VERY EASY...