Changing Mortgage interest to New PPR

[pennypincher]

I'm in the process of selling my first PPR,last year I bought a bigger PPR,I did nothing with the first house just left it there and stayed a few nights if I got stuck in work etc.Anyway I'll have the first house 5 years in September but I'm due to sell it the last week in August.I never changed over the mortgage interest relief onto the Second PPR as for the sake of a year it seemed too much hassle.What do I do now once I sell....is it just a matter of telling revenue to change over?