Sale of Spanish flat

[Brightonian]

My client, who is UK resident, sold her Spanish flat last year. I am unsure as to how to treat the Plusvalia Municipal she paid. This is a local tax and, according to the DTA, local taxes on capital can be offset against UK CGT. So far, so good.
However, on researching further, it seems that, in Spain, it is treated as an expense for the purposes of their CGT.
I am therefore unsure whether I should be deducting it as an incidental cost of sale or setting it off against CGT. I am inclining towards the latter but am not 100% sure.
Has anyone come across this?