Not quite, Maat.
Sub of £310, tax relief of 40% on 67% of it:
310 - (310 x .67 x .4) = 227 = £19 pm, £38 pm in year 2 onwards
For a F/O on say £36000:
180 - (180 x .67 x .22) = 132 = £11 pm, £22 pm in year 2 onwards
Cheaper than most of us might have thought, I would agree.