Mountain bike rain coat / jacket (packable, not for winter) I am looking to get a thin rain coat.  I have a good one for the depths of winter (it needs water proofing again), but it is way too hot to wear it any other time. (I am a portable radiator.)  My hope is that it would be thin, so not too hot, and that it will pack down into a small size so I can store it in my bag whilst riding. (I like to do long rides so may not want it all day.)

I do 70/30 offroad to on road riding, as much off road as I can get but I don't have the best selection of hills at my door step.

I am happy to build up my layers so that the coat is only there to stop the rain.  Main points I guess are:


stops the rain
will not get me too hot
packs away small for transport when not needed.


I swear I saw a good review of half a dozen or so such jackets a while ago, but for the life of me I can not find them anymore.  Does anyone know of a good jacket, or even better, links to good reviews to be able to look at the best options?
Recently introduced by 'Altura' (the poor British man's 'Endura') is the 'Pocket Rocket':



This jacket is distributed by Zyro and widely available in British bike shops. Visit a stockist and try for size wearing what you expect to be wearing underneath, i.e. more than a T-Shirt. Remember that when buying from the LBS you may pay RRP rather than 'online price', however, you won't pay postage, have to wait or be bereft of a coat if a seam goes and you need to take it back. (Altura seams are pretty good b.t.w.)

Available in black, yellow or red versions of hi-viz, the reflective parts of this jacket will dazzle on-coming motorists. A neat feature are the thumb loops that enable you to keep the sleeves tucked under your gloves.

IMHO a lot of 'breathable' does not work, so there is no point bothering with it. There is some rating of breathability for this jacket, but you can expect some 'boil in the bag' aspect. Waterproofing is claimed 100%, just what you asked for. 

A pack-to-nothing jacket is truly useful to just have. It is also nice to have a relatively plain jacket without any fancy colours or extra seams. This jacket keeps it simple. The pack size is exactly what you want, i.e. very small:



You can dangle it on the handlebars if you are in casual clothes and without a bag. If you get this jacket, expect people you know to want to permanently borrow it. Also expect to see other people on bikes with a jacket remarkably similar to yours.