Advice, comments please,newbie

£150/month is 3.6% of £50K. This should be fairly easily achievable. Where things start to get more difficult is if you want this figure to be index linked. This requires your capital to increase by inflation as well as generating 3.6%. This level of return would require you to accept some moderate level of risk as it is only equities (shares) that can provide a long term good chance of at least matching inflation.

The other question is whether you would be prepared to accept some level of variability in your drawdown. You really need this if you are investing in equities as during the bad times you don't want to be forced to sell the core of your investment thus reducing the capital required to generate future income. The easiest way to manage this is to hold a reserve in cash to take as income when required.

Do you wish to 'protect your investment' to pass on in your estate or are you OK to draw capital with the aim it could reduce to zero over your remaining life?

I agree with Linton that investing could generate the desired returns (but I am more in the 3% rule camp especially for early retirement) and I would probably use a low cost stocks and shares isa which would give a tax efficient wrapper to £20k pa (or less if you have contributed to your cash isa this year) maybe also using your wife's allowance to get it wrapped quicker?

Investment comes with some risk and volatility and unfortunately the traditionally low risk bonds/gilts are looking a bit high risk these days. In your position I would open a Vanguard direct ISA in the Life Strategy 60 fund and draw down £100 per month initially increasing to £150 per month if the investment performs well. Or if you want more money in early years start at £150 and be willing to drop back to £75 if you start to see too much capital errosion.

Also up to the age 75 (or until the money runs out) then I would withdraw 2880 per year to contribute to a private pension and the government will boost it to 3600. You might be able to contribute more in the first year matched against your earnings or use your wife's pension. It doesn't matter if you are withdrawing from the isa when markets have dropped as you will be buying in the pension when markets are cheap.

Make sure you are maximising the use of your annual income tax allowance and tax free element each year. Also check there are no missing NI contributions you could make to get full state pension.

Finally - how did you get your wife to agree to retire after you? Mine is 6 years younger and wants to retire no later than me!

There are multi asset income funds with fairly low volitility which might suit your risk level.