I have seen two potential plots that I really like the look of and both come with full planning permission.  I am sorely tempted to put in an offer on one of them if I can get a mortgage for just the land.  Do mortgages on land then come with a time limit on when I would have to start building or can I leave it a year or two until I get more funds and do more research to see if I want to change the current building plans?  I realise the PP comes with a time limit but I'm not sure about the mortgages.  Is this perhaps one for a mortgage broker who specialises in self-builds?

Try Buildstore https://www.buildstore.co.uk/mortgages-finance/project-types/self-build-mortgages

and Ecology https://www.ecology.co.uk/mortgages/residential-mortgages/self-build/.  They do lots of mortgages for members here.