So here's what happened...
I took it in for a battery test and it said everything was fine, no issue with the battery, no issue with charge coming from the alternator!
I got myself a voltmeter, watch some youtube videos and went at it. Low and behold the battery was fine, but when I put the electrics under load by turning absolutely everything on, the battery light starts coming on intermittently and the voltage coming back to the battery drops from 14v-ish to just over 11v.
As I had acted prematurely and got the alternator already and backed up by the voltmeter I then watched a few more youtube vids, grabbed my cheap Halfords sockets and went at it!
3.5hours later the old one was off, the new one was on but I just couldn't get that damned serpentine belt back on, went to bed and slept on it. This morning I tried again unsuccessfully for 3 hours before giving up and heading back to youtube only to discover I need to remove the wheel arch lining to get better access, a few minutes to do that another 20 to get the belt on, then it all went back together.
Got my new favourite tool out (voltmeter) and started probing. Problem solved
getting over 14.5v even under full load.
This was my first ever alternator, in fact, I've only ever done brake pads before. And aside from having lost almost all feeling except a constant stinging sensation in both hands from all the scratches, nicks and cuts I earned along the way I delighted with the outcome (and saved over £200 from the price quoted by the garage).
Proper chuffed with myself and all thanks to all your advice and suggestions so thank you one and all
Now onto the next job... passenger side door has now decided not to open from the outside, insides fine!
