You call yourself a UK Newbie, and it just so happens I'm in the same boat. What I've done so far is set up an S&S ISA with one of the direct-investment places (I'm at Interactive Investor, but I'm told Cavendish have slightly lower trading fees), and the ISA contains a Vanguard LifeStrategy fund. You get to choose the rough balance of bonds/stocks you'd like, and Vanguard do the hard work of rebalancing the particular components for you.
So that gets you to £15,240 per year (the allowance rises each year); your mortgage is indeed not at a high enough rate to pay down instead of investing the extra, so you might want to look into setting up a SIPP or opening a taxable trading account, and putting more Vanguard in those. You're probably already aware, but a SIPP or other pension scheme can't be accessed until you're 55 years old, which may affect your decision as to whether to keep much money in there (I don't have a pension at all, for example).
Hope that helps