Start/Charge fault

[CraigSRi]

Recently I got my J back on the road after it had been stood for a year without being started much.
The car had been running fairly well for the first week or so so I decided I was gonna treat it to a full service, replace the thermostat (had the "service vehicle soon" message for some time), coil pack, get some new floor mats and take care of the minor advisories on the MOT.
For a week I noticed that the car was starting to get very sluggish turning over, but it always fired up. Eventually one day in a petrol station it failed to start so I was lucky enough to get a jump start to get going. Once home the starting issue turned into nothing more than a click on startup.
Jump starting wouldn't work, I could only start it with a push.

I ordered a new Varta Blue battery and it was delivered next day, but despite the new battery being fully charged the car would still only click and not turn over.
I took advice from a few people and also did a lot of research to diagnose the problem, and everything pointed to a faulty starter motor.
I booked it into a local garage and today they tell me that there is no charge going into the system, the tests showed there was no charge coming from the alternator.
(On the way to the garage I got a new message, "Service battery charging system" and the battery light stayed lit)

So anyway I'm told I need a new alternator, is this likely to be an expensive fix?
This year I plan to trade the car in so I'm not really inclined to spend a fortune on it, so should I go for an OEM quality part or pay the extra and get genuine?

Apologies for the wall of text, and thanks for any replies.

[Rusty2009]

If you are planning on trading in I would go pattern part have a look on GSF they have 56% off until the 18th using code GSF56

[CraigSRi]

If you are planning on trading in I would go pattern part have a look on GSF they have 56% off until the 18th using code GSF56

Thanks Rusty I'll check them out and see if buying the part and having it fitted compares well to the quotes I have (for both supply and fitting) had so far. Best quote came in locally at £290 which seemed pretty fair to me. Would be great if I can get it cheaper still