1. I'd like an antenna that can do 3g now but will I need something for 4g so is there an antenna to do both?
4G should need two antennae. Actually we only have one and it works, apparently it shouldn't. I might be able to speed mine up with two antennae. But, rather than have your roof looking like some sort of transmitting station you can get dual antennae "in one".
These guys are quite helpful:
http://www.solwise.co.uk/3g-antenna-outdoor.htm
I wonder if they might let you return it if it doesn't work out, because you need one with a good db uplift so the cheaper ones (e.g. +5db or less) are likely to be little use.
You can get the information about your connection from the B593 - System > Diagnosis > Wireless Status Check which will give you something like:
PLMN: 23430
2 Service status: Valid service
3 RSSI (dBm): -57
4 RSRP (dBm): -84
5 RSRQ (dB): -8
6 Roaming: No
(This is our EE 4G service)
Solwise may find this information helpful to assist you.
The issue that you have is the hilly terrain which means a directional one may not help unless you can get it high enough.
Height is probably going to be everything here. If you feel adventurous you might be able to climb up a ladder with the modem on a power lead and see what stats you get there
In other words, if it were possible to put the B593 at the top of your chimney then that alone might be more effective than the most powerful antenna at the same height as you have the modem now.
(Just to illustrate the point, not suggesting that you do fix the modem to the chimney outside!)
2. I am with Bouygues Telecom. Does the antenna need to be specific to their signal?
No, but it has to support the frequency range/set - look that up and compare with that stated for the antennae on the Solwise page.
3. Mobiles don't really work for calls or data inside the house due to thick walls; is it possible to buy something (so a booster rather than an antenna) that improves the cell/ 3g signal generally rather than an antenna for the Huawei only which will then be picked up by both the huawei and phones inside the house, helping me in two ways from one purchase? Probably a long shot but it would be a double win if there is.
No. I'd like something like this. We can get great MBB but we can't have a reliable mobile phone service in this house. But we can use VOIP through the B593 which is flawless.
You can sign up for a service some mobile providers offer which uses a landline to route the signal, but that's not quite what you were asking. Since we don't have a landline I haven't tried this.
4. And finally, can I assume that placing an external antenna high on the roof facing towards the mast is the best place for it?
Yes. As before I suspect that height is going to be everything here.
Let us know how you get on.