On a 55 plate 2006 forester 2l petrol

With factory unit 6cd changer type

But it doesn't have mp3 compatibility or aux option?

Someone suggested the possibility of adding a Bluetooth dongle to the rear of the existing unit ! Is this an option?

Or is it possible to buy replacement stereo units with all the functions of our modern life?

A replacement unit is an option although dependant on model you may lose any steering wheel controls. Not heard of the bluetooth dongle option in the rear but dependant on what input would be connecting it to there may be a cable option also

I use a Bluetooth fm transmitter, got it off eBay it's called a X5. 

This is the radio unit type

With no AUX setting that I can see I'm unsure where I would stand with a BT unit

And also not sure where to start to replace the unit?

looks like a double din unit, food for thought but there are many to choose from with differing styles and functionality

https://www.dynamicsounds.co.uk/kenwood-dpx-7000dab-cd-car-stereo-dab-radio-bluetooth-usb-aux-ipod-iphone-player-aerial.html

You may need one of these?

https://www.dynamicsounds.co.uk/subaru-steering-wheel-interface-adaptors.html

Hi seen your thread this might help ,just fitted GROM unit from grom audio .brilliant  hands free phone bluetooth and took less than hour to fit cost £109.other options were kits for memory sticks etc .might be worth a look on site 

Thanks everyone for this

As time permits with work), I'll look in to this and report back

Grom link

http://gromaudio.co.uk/partfinder/product/list/?make=Subaru&model=Forester&year=2004-2014 (check radio model number)