Advice please, about to retire, no pension, but with a lump sum due

I’m 65, will retire next year, working in same company for 42 years, also company md, but no shareholding. Company has always struggled, hence no pension and underpaid for majority of time working. Company now due to partial sale able to pay
How best can I maximise my lump sum… thanks

What lump sum?

€800,000

Lump sum for/from what?
How can anybody suggest how you can maximise it when they've no idea what it is, where it's coming from, what terms and conditions govern its payment, and what leeway you might have for negotiating the amount payable upwards?

If you mean that the €800K lump sum is what you're going to get regardless and you want to know what you might do with this once you receive it then you'll probably be better off doing a Money Makeover post to give more detailed background on your overall financial and personal circumstances.

It’s a termination payment… how can I invest in a pension, is there retirement scheme that might reduce tax etc etc
There’s no revision upwards

You need to get a clear definition of what that lump sum actually is so you become tax compliant on the payment itself. For example, if they are paying you redundancy and you have worked 42 years, then potentially the statutory redundancy element of that is tax free (which as MD and assuming you were earning more then €600 pw would work out roughly at €50k). Likewise, there are other tax calculations that are done on an ex gratia payments but I would suggest you start with a good accountant to understand what those are and mean for you. You will have to pay some tax on an €800k redundancy payment.