Last year I did some work on my garage and ended up getting my main fusebox replaced.
Mine was like yours is now.
(see linked thread, page 2 near the end for details)
http://forums.overclockers.co.uk/showthread.php?t=18675262&highlight=Syla5&page=2
You can see I also had the main fuse, and fuse carrier to the house replaced. and the tails (cables out of the fuse in to the meter) replaced. All of this stuff is the responsibility of the power network (not your provider, but the company that maintains the network its self). I also got the energy company to put in a self isolator so that the electrician could turn off the mains without the need to contact the power company.
If your fuse board is like mine it will only be rated to 60amp, and if your main fuse (the green thing) is anything like mine it will only be a 60amp as well. Get in touch with your energy provider and ask them about who you need to contact to get the fuse carrier and fuse replaced. Then get in touch with them to book the change and fuse upgrade, they should stick in a 100amp main fuse if you ask nicely. (I had to contact UK Power Networks, very good customer service)
Once the main fuse and carrier has been replaced get back in touch with your energy provider and say you want a "self isolator" putting in (also free to you, and at the energy companies cost). This will mean that an electrician wont need to contact the power company and can just flick the mains power off themselves. I know most probably don't bother and just cut the fuse tags and pull the fuse, but with a self isolator they are not breaching any regulations and so it makes things smoother.
After that a replacement fuse box should be somewhere in the region of £200 to £300 give or take and is a relatively basic job as long as your wiring is up to standard and doesn't cause the new RCD board to trip (residual current detector - trips out much faster then a standard breaker fuse) .
To save time on all this I booked in the main fuse carrier replacement and as soon as that was booked in I booked in the energy provider to do the self isolator within the same week. Doing this saved me 3 weeks of waiting between the fuse/carrier and self isolator being done.