Cash in your mattress time...

"Have you actually got more than the government guaranteed 50k to save?"

you are missing the point. inflation (real inflation not the government statistics) and the drop in interest rates means that your savings (especially if they are not tax free) aren't really doing much apart from remaining static.
we are supposed to be prudent and save not spend recklessly with easy credit yet are penalised by doing so.

Silly me, it'll far more likely earn interest as a fire hazard under the mattress or in a price fluctuating commodity of precious metals.

It may remain effectively static, it will also remain safe. Other financial advice really should be personalised to the individual, such as mortgage overpayment (banks quire interested in this at the moment) and fringe investments.