It looks like the 5 hours are part of the 38 hours, but worked outside 'normal hours' and therefore attract an enhanced rate, which equates to £10 an hour.
The calculation should probably be something like;
33 hours @ £25 = £825
5 hours @ £35 = £175
I am assuming that there is an agreement on how 'overtime' hours are dealt with under the contract, what constitutes 'overtime' and how such hours are 'approved' by the PM..