Hi Helloyellow. There is more than one way that these things could be treated so, whilst I can give you an answer based on my own view, the only view that really matters is your trustee's.

In respect of your income. If someone's income goes up and down regularly then yes, an average over a period of at least 3 months would be a reasonable basis for the calculation. If your reduced pay in October was a "one-off" then I might treat it like that and base your contributions going forward on your normal wage instead, with an allowance made for a reduced payment just for that particular month. It is really just about trying to asses things in a fair way that makes sense.

In terms of the expenditure items you mention, it really depends on a couple of factors.

Firstly, would allowing additional spending mean that your overall expenditure is higher than the guidelines that trustees use to assess what is reasonable? It is not something that you could know, but your trustee would and might limit your allowed expenditure as a result.

Secondly, would allowing these extra expenses mean that your payments to the Trust Deed would have to be reduced to a lower level than what was originally proposed in the budget you agreed to at the outset? If so then again it may not be something your Trustee would sanction.