Wi-fi signal increasing ideas?

Hi,
Ive got a wifi router upstairs its a vodafone hg556a and i want the wifi downstairs,
At this point i will explain i am fairly experienced with wifi and home networks,
I just want to know if anyone has any experience with this particular problem.
The router is above the room and in the rafter is 300mm of kingspan foil backed insulation, foil backed insulation under the wood floor, and some rubber sound deadening material.
I unfortunately cannot use the following options:
1. Wireless access point connected to the router via ethernet
2. ethernet over cable, as there are two seperate fuse boxes for upstairs and downstairs

Downstairs my Galaxy s2 can pick up some wifi directly underneath the router but i want it a bit better. The wifi analyzer app shows a fair to good connection and states that the wifi speed from the router is about 5-13mb which is kind of crap, I know the dsl speed i get hovers around 4-6mb, but i just want the wifi range to be a bit better and more realiable.

I want to know if anyone has any ideas on the following things im considering as an improvement.

1. With the router unmodified and aimed downwards the signal directly underneath stays at about 13mb, so im considering using a wifi signal booster to at least have a decent 13mb link between the router and wifi extender with good signal downstairs. I am aware that repeaters can slow things down as it repeats the signal, stores packets and repeats them again.

2. Upgrade the antennas, yes they are internal antennas but ive added SMA conenctor before to upgrade antennas in the past.

3. Use a directional antenna between the router and wifi repeater but it may reduce the signal upstairs, although it should not matter much if it does.

Anyone have experience in this or any other possible solutions?
Thanks guys and gals