Its all been going so well until yesterday. took a quick trip of about 5 miles with all the heating, rear demister, fan, AC, and the heated seats on, its was bloody cold, parked up and switched off engine. 2 mins later tried to start and just a high speed ticking. Being an Automatic cant bump start so called out RAC. They did turn up and he boosted the battery to start her up.
OK
Not being a tech mechanic or any kind of mechanic, what is wrong?
Voltmeter shows with no electrics on over 14v with engine running, turn on fan, lights it drops to 12.5v turn on more and its in the 11.5v area. RAC man said that the alternator might be dodgy (after he put a metal rod on it and listened) or as the battery was for a 2ltr petrol it could be that, oh the positive terminal was loose but now tight.
Never had this issue and the battery has been on it since I got her 6 months ago.
Would the correct size battery solve it or a stupidly expensive alternator change?

The battery voltage should hold 13.8 volts with everything turned on. So if it's not, it suggests the alternator is at fault. It could be just the brushes that need changing, as alternator windings are generally very reliable. Sometimes the bearings fail too, but if they're quiet, then I see no point in replacing an alternator, simply because the brushes have worn out.

The TD4 alternator is unfortunately very vulnerable from oil contamination when servicing, because it's under the oil filter. I've seen loads that are swimming in oil, because the spanner monkey couldn't be bothered to put a plastic bag over the alternator when changing the filter. This oil often finds its way into the casing, causing bad connections and brush wear.

Cheers, would a spray with WD40 or an electrical connection spray help or is that pointless. I guess the next question is how the hell do you change the brushes?

No. WD40 won't help. The brushes are available for the Veleo made starter. These don't need the starter to be removed, as they are on the end of the unit. It's just a few 13mm bolts iirc and a plug.
Changing these will often cure the problem, but it's not a guaranteed fix. I'd remove the brushes first to check there condition and length. They should be about 15mm long, but often wear down to stumps, especially if the alternator has been doused in oil.

I don't have the kits to do the job myself so sadly will have to get local guy to do the work. Might just get a new one fitted!!

You're obviously getting some power from the alternator, so it could be something simple like the drive belt slipping.

If only they put it in a simple to get at place!!