Bit of background: I am 41 and presently employed as an R&D manager for small company (I've been doing similar roles for 18 years). I also only have a BSc hons and no masters. I was looking into returning to academia by doing a PhD and moving forward from there. Is 41 too old to be considering this and would the lack of masters be a significant hindrance.

Of course not. If you like to do a PhD then go for it. I'm pretty sure you're not the first person to do a PhD with that background.

You have more chances of doing a PhD in a STEM field without a master's. I'm not sure about humanities fields though.

It's tougher to get a PhD position without a master's but it's not possible.

I suggest you contact the university you wish to enroll in for the entry requirements.

41 is definitely not too old and the Honours degree probably won't be a problem as long as you have a 1st or 2:1.
Good luck.