How to invest extra income, should I pay off my mortgage or is there a safe way to ininvest with better returns?

I've recently stopped drinking which has resulted in both a big drop in expenses and an increase in income.

I have a 10 year fixed rate mortgage at 3.9% and about £98k left on the mortgage. No other debt and about £20k in savings.

However I have no pension, I work in the construction industry so I've never had anything like employer pension contributions, holiday pay, sick pay etc. Also my parents payed into an indemnity pension for 25 years only to be told it was worthless, so I don't really trust private pensions.

My plan was to buy a place to renovate and rent it out, but that no longer seems feasible. Basically I get paid more for renovating other people's properties than I'd get on any equity increase on one I bought and refurbished. Also with the taxes, stamp duty etc a buy to let seems like a bad investment.

Should I just pay my mortgage off? Or is there a safe way of investing the extra savings/income which is about £1,500 a month, where I can't get screwed out of it.