£40,000 secured loan against property... easy buy scheme

hostie

If you are still with your original mortgage with Nationwide, you could look into shopping around for that part of your mortgage now. You could either get your monthly payment to be less with another mortgage provider on the same amount or you could look to increasing the amount by say, 20k. Then put that aside, so you know that you have half covered, and in a couple of years, try to re-mortgage again for the other 20k or if that fails, take out a loan for the other 20k.
At the moment you are paying capital payments on the £120k, but depending on your age, you could potentially have 20 years to pay the capital off. May be worth switching to an interest only mortgage for the next few years and putting the money that you would use for the capital part aside to cover the loan.
You are lucky that you have started thinking about it early. You have a few years to find it and there are two of you so if all else fails you could try to up your income. There are plenty of threads on mortgage free wannabe and debt free wannabe with inspiration for making extra money.