Answering your cycling questions

It doesn't have a charge port but Exposure sell a set of C123 batteries plus a charger, similar to normal battery chargers. To be honest, the batteries are pretty easy to find and aren't that expensive. Exposure's own data shows a burn time of about twice as long for disposables compared to rechargeables. Unlike the Flash front unit, there's no difference in light output when using the different types of batteries. I'd stick with the disposables.

yes, there is a recharge kit, which we sell for about £17.99

Yes, it can be if you use CR123A lion battery and charger. More importantly, you can change the battery when its duff - it isn't propriety.

You need to purchase the recharge kit and batteries for around $18 to £20

It comes with a disposable battery. You can purchase a rechargeable battery kit incorporating a mains charger and two rechargeable batteries, this is quite a big additional expenditure, although it will probably be less than disposable batteries in the long run.

Alternatively you could go for a fully rechargeable rear light like a backupz USB, although this type of light is bright, they're not as bright as the Flare.

If you own an Exposure front light with Smart Port Technology you could buy the Red Eye (forerunner to the Flare similar spec) which is charged from the front light. It comes in either seat post or helmet mounted options. It will reduce the battery life of the front light though and involves cables.

Only if you purchase the Exposure rechargeable battery pack and batteries.

It is supplied with a disposable battery though you can order rechargeable batteries and a charger at additional cost £15 - £20

This unit when purchased by itself (£39.99) is unfortunatley not, but the 3v battery included is of very high capacity (1300mAH).
This will give you over 20Hours in flash mode
8 Hours Constant.
You can purchase rechargeable Lithium rechargers & cells (RCR123A) online, but unfortunatley have about half the capacity.
Also lithium batteries can be extremely dangerous if mistreated (over charged / over used), so make sure you do some research if you are sourcing your own charger and cells.

this light unfortunately can't fit on a helmet. we've tried it and tested it and broke two in the procress.

I looked at that, but when I got it home, in my opinion, I would go for a specific helmet light that sits closer to the helmet. This would stick out quite a bit..

This light is designed to be mounted on the seat post. You might be able to fit it to some helmets depending on their design. I'd try it in a shop first before you buy.

There is a version of the Exposure Red Eye (similar performance to Flare) which is specifically designed to be fitted to the helmet mounted Exposure Joystick or Diablo, but that's not much cop if you don't already own one of these lights as their not cheap.

Not with the bracket that is supplied it isfor the seatpost.

No, its not designed to fit a helmet, however, depending on your helmet design if you have deep rear vents, the silicone strap may be able to stretch around it.

I bought the standard one, it you probably can as you would just have to feed the silicon through one vent then out through another.
The problem would be aim, but I've found generally flipping the unit aims it high or low.

Yes

they are now back in store, snap one up.

they are now back in stock.

I wouldn't have thought it would work very well on an aero seat post but I'm sure it could be made to work with a little rubber and electrical tape.

CR123A (or rechargeable RCR123A) batteries are around 32-33mm long (without/with +ve terminal nipple) and 16mm in diameter. This makes them about 2/3 the length of an AA battery and a bit fatter.
(The rechargeable ones need a specific charger - an AA/AAA one won't do).

I'd say it would work. The flare would still do the job even if directed up. You could always fashion yourself a shim or sorts.

Or use a cut up peace of old inner tube to make the seat stay fatter.