Lump sum

With no risks you are talking savings only.

Some may argue further to minimise your risk you need to split it into 35K chunks in case any bank/BS collapses you'll be protected for the full amount.

You'll get the best rate on a fixed rate account or "bond", these are over 7% at the moment (Birmingham Midshires will have one at 7.17% from the 4th July), but you'll need to lock the money away for a year.

So you could split it, some in fixed rate accounts, some in high rate instant access accounts (Kaupthing Edge, Icesave etc), and also you might want to look at NS&I index linked savings certificates, you can put 15K away in a 3 and 5 year term, so that's 30K "invested", here you get a interest rate + a link to the RPI rate of inflation, which is very good at the moment, and it's tax free.

The highest rated accounts are listed at Moneyfacts.