Could just be me being a tad paranoid but ive noticed my main battery is showing fairly low. Usually around 13v when driving, heated screen/seats etc... drops down to ~12.8 or so.

Showing 12.4v when stationary

Aux battery is showing 13.84v when running 12.45 stationary.

Main battery is about 2 years old, maybe 3.

Could the alternator (original) be on the way out?

Cheers
Josh

Doesn't sound too bad to me. Does it go as high as 14V just after start, if so then the alternator is probably fine.
If you are worried about the battery you can buy battery testers quite cheaply. I have one from Foxwell that works well. You connect it, tell it the type of battery and the CCA it is supposed to have and it will give you the state of charge, the state of health as a % and a clear statement of advice as to whether to change it.

Of course, didnt think of that

Just being dim....

But no doesnt go above 14v

A healthy alternator should be able to deliver a little over 14V to a battery that has just had a big discharge - starting the engine. Your alternator might be a bit tired, but it could easily work perfectly well for a few years yet. Worth checking it though.