Mortgage for a self build in Northern Ireland

reg79

Hi, I posted this on the Northern Ireland forum yesterday but I think it would be more appropriate here as there seems to be a few mortgage advisors lurking.

I am currently in the process of trying to buy a building site in Northern Ireland. The site has outline planning permission already approved and I would appreciate some advice as to how to proceed with the purchase.
I have a substantial cash deposit that will amount to approx £140k, the asking price for the site is £170k meaning I will have a shortfall of about £30k.
Should I go ahead now and try to get a self build mortgage to incorporate the cost of buying the site and building the house (Estimate that will take approx £180k) or get an alternative form of finance to pay off the site and then take out a self build mortgage?
As the land has outline planning permission it would potentially be 12-18 months before I would be in a position to build anyway taking into account the length of time it would take for me to obtain full planning permission and get a suitable builder to start the job.
My current salary is 32k and my partners salary is 15k. We currently have no credit card debt, car loans, mortgage etc.
In the past I have used an independant mortgage broker who has been excellent, I ran this past him a few weeks back and he thinks my best bet is to pay the site off seperately over 1-2 years and subsequently arrange a self build mortgage as the entire market for potential lenders would be open to me.
Has any one any advice or encountered a similar proposition?