If you want a retirement income of say £40k how much would you need in the bank to earn that each year with relatively low risk?

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right now? - about 1 million

you'd have to actively sweat a million to get 40K out of it - a friend of mine has a country house of that value that they use for very short term lets (wedding parties etc) and get pretty much that revenue, but it's active work and carries risk. leaving it in a bank gets you squat these days, and giving it to a wealth manager to play with wouldn't let you sleep at night...I've always reckoned you need 3-4 million in the bank to retire and continue having a decent lifestyle

are we talking £40k per annum??

It depends greatly on the age of the retiree. an annuity is based on age, life expectancy, fund value and interest rates at the time of annuity purchase.A 65 year old would get 40k out of a million no sweat. A 40 year old might struggle.

Start smoking now to bring your life expectancy at retirement down. You'll have a better chance of meeting your £40k/annum objective.

after retirement I intend to outlive my wife then get myself a 25 year old Ukrainian off the internet. How much a year will I need in order to convince a good looking one?

A 40 year old might get a 25 year old out of a million. A 65 year old might struggle.

Yet a 75 year old millionaire will have a better chance than a 40 year old, and a 95 year old even better.