My potential new manager is asking sensitive information from my references [closed]

I'm applying for a job in Marketing in England. My potential new manager asked my age on the phone interview. I let it go as he is not originally from here.

But it got to a point where he is now asking my former bosses for things including:

• My salary
• Whether I was fired or left (I left),
• Whether they would re-hire me, and if not why
• How 'trustworthy, reliable' I am etc.

I thought this was a good opportunity as I have been unemployed for some time now, but am having second thoughts. Is it okay for him to ask these questions? Is this a red flag for what it will be like working at this company?

Age

Asking your age is only allowable if the job requires you to be a certain age to carry it out (e.g. selling alcohol or obtaining certain licenses that have age restrictions) - as you mentioned that it was a Marketing job this is probably unlikely to be the case (not impossible though)

Salary

They can ask but as the answer to this counts as personal data, your previous employer shouldn't really answer this without your consent.

whether I was fired or left / whether they would re-hire me / how 'trustworthy, reliable' I am

These are perfectly reasonable questions to ask a reference and there's nothing illegal or unethical about asking them.