Coat suggestions

Have you got full length mudguards?
Biggest difference in my opinion as the majority of wet days it is just surface water rather than from above.

Jim, the Castelli jacket/jersey (I forgot the name) is apparently so good that non-sponsored pro riders were wearing them in the cold early season races like MSR.

Ah, found it! Gabba.
http://road.cc/content/news/78651-milan-san-remos-other-winner-castellis...

Jim, I think that you have got it right. I ride about an hour each way on my commute and I find that a good soft shell windproof does the job on all but the rainiest days. A decent windproof will be fine in drizzle or a shower and will be much less sweaty than a waterproof. I save the waterproof jacket and trousers for downpours, I get pretty hot and sweaty in there but it's much better than cold and wet. I like Gore windproof kit but it may look too cycle specific for you.
Simon's earlier recommendation for a cycling cap is a really good call too. A cap will keep your head warm on cold days and does a great job of keeping the rain off when it does come down. A simple cotton cap has really made my winter riding much more comfortable.

The howies's thinner NBL Light and Endura baabaa are lovely under a windproof but not unless the temperature is in single figures - and nearer zero than 9.

I looked at packable jackets from howies and endura too, both £40 IIRC. I have a Gill waterproof jacket for heavy rain but it gets hot in there! Something showerproof, packable and more breathable is better for me 98% of the time.

Tried overtrousers but I think I'd rather get wet legs. Considering some roubaix tights or legwarmers with a coating like Lusso Max Repel or Castelli (Nanoflex?).

Give it a go with your current jacket, cotton just traps sweat. Howies, Icebreaker and Rapha do good quality merino baselayers. I have heard good things about Torm too, though not tried them myself (torm.cc)

Short or long? Long is probably more use under a jacket, but I have both for different temperatures. I wear two under a softshell when it's really cold.

What do you wear under your jacket? Anything cotton will make you very sweaty. Merino base layers are good.

Sorry, can't help re. a coat. I get hot easily so choose the lightest, thinnest layers I can get away with and sod the appearance.

I find full-length mudguards, waterproof overshoes and a cycling cap (whether you wear a helmet or not) make riding in wet weather more pleasant.

Also important:
- good lights
- p-resistant tyres
- well maintained drivetrain and brakes
- confidence in traffic / road skills

if you can't afford a showers pass one then surface do one called the aquaphobic, we found the fit a touch odd when we tested it but it might suit
http://road.cc/content/review/50600-surface-hydrophobic-wool-jacket

Have a look at the showers pass amsterdam too. i've got the portland, which it replaces, and have had for over four years. it's a super jacket.

likely to be £150 when it lands over her though, possibly more

If you can fit into a small I would reccommend the Rapha City Rain Jacket. £135 in the sale until Wednesday with the extra 10% code. I hear good things about the hardshell jacket too, only in small on the website but I did see a large in the London Cycle Club at the weekend.