Sounds a good start , sound like the gaps round the joist are the main problem
you could you use something to parge coat (wet under coat plaster) the blockwork between the joist to help seal hole area or perhaps foam fill the gaps to get a longer lasting seal, decorators caulk has a tendency to shrink back a bit
Over the garage if you haven't already insulated you could perhaps use a better insulation , such as PUR board , though this requires more detail work to get a tight fit
I presume you've had your cavities filled

cheers Jim

http://www.energysavingtrust.org.uk/business/Business/Building-Professionals/Helpful-Tools/Best-practice-house
http://www.energysavingtrust.org.uk/uploads/documents/housingbuildings/CE83.GPG155%20-%20Energy%20efficient%20refurbishment%20of%20existing%20dwellings_4_01_08.pdf
http://www.energysavingtrust.org.uk/scotland/Global-Data/Publications/Advanced-insulation-in-housing-refurbishment-CE97

take a look at these for more info.

thats a good photo to demonstrate the problem of air leakage round joists and wiring
have a look at this guide for all round energy saving advice and costings
http://www.energysavingtrust.org.uk/corporate/Global-Data/Publications/Domestic-energy-efficiency-primer-CE101-GPG171

if moneys tight,stick to the mineral wool for insulating BQ/wickes etc they have lots of 2 for 1 deal as I'm sure you know

Get your cavities filled asap , that will massively improve the problem . Phone up now and get the ball rolling, there may be a long waiting list
I presume you have a 3 bed + house as you have a bedroom over the garage
its approx £200 to do and the pay backs less than 2 years so if make sense to borrow money to do it .
Stick it on a 0% credit card if you can , pay it off a the end of the deal , Its at least 50% return on your investment so its a no brainer
what bank offers that kind of return on your investment?
they'll stick some more insulation in you loft too for £150-200 but I'd suggest if possible doing it yourself now, or next weekend at least!
the average homes about 50m2 and you should be able to get the materials for £150-200
get it in for this winter , your get you money back in fuel saving before March
if possible go for at least 300mm in the loft
also look into draught proofing all doors,windows, cable/pipe entry points, basically anywhere something creates a hole into the cavity
imagine your house as a balloon and if you dont fill those holes all the air (hot) going to whiz out

here some links to cheap cavity fill and insulation
http://www.diy.com/diy/jsp/bq/nav/nav.jsp?linktype=breadcrumb&fh_location=//catalog01/en_GB/categories<{10082}/categories<{10088}/categories<{9231012}&&ts=1223555266608

this stuffs nice to work with and doesn't fill your loft with nasty fibres so you can still store stuff and move it around a bit to suit.
http://www.diy.com/diy/jsp/bq/nav/nav.jsp?action=detail&fh_secondid=10186706&fh_view_size=6&fh_eds=%3f&fh_location=%2f%2fcatalog01%2fen_GB%2fcategories%3c%7b10082%7d%2fcategories%3c%7b10088%7d%2fcategories%3c%7b9231012%7d%2fspecificationsProductType%3dloft_insulation&fh_refview=lister&ts=1226261399357&isSearch=false

What is the point in insulating the garage ceiling if wind can get into the void above it?

Dont rely on cavity insulation to stop drafts, you need to physically seal them out of the warm parts of the house.

Well done with the loft insulation.

Celotex on your garage ceiling would be easier than pulling up the floor , but if you're pulling up the perimeter to deal with the gaps into the cavity
could you feed the fibreglass in this way ?
Celotex is expensive , 100mm 8/4 sheet £33+ ( this is approx. the same a 200mm mineral wool, I think you'd need to line over it for fire proofing
either 2 * 12.5mm plasterboard or 12.5 fireline
you can get plasteredboard with the PUR celotex/kingspan etc lamenated to it , but again its not cheap ( more than the 2 seperate items )

Jim