Howareya,
I think you're getting two different ideas jumbled.
A Self Administereted Retirement Trust is a form of pension fund, whereby you make your own decisions as to what assets your pension fund holds, rather than leaving such decisions to a pension fund manager. So you have a pension fund accumulated in the normal way (through contributions and/or a transfer from an existing scheme) and you decide that your pension fund buys a property as an asset. It's important to note that the pension fund owns the asset and not you.
This is totally different to the idea of a pension-backed mortgage. Using a pension-backed mortgage, you get an interest-only mortgage to buy a property personally. The original sum borrowed is repaid from the proceeds of your pension fund when you retire.
There are loads of threads on Askaboutmoney discussing both self-administered pensions and pension-backed mortgages.