Help with noise - whine/roar rear or car

My MX5 has been making a noise for quite some time but it is getting worse. It is like a wheel bearing noise (had this on the Octavia at the front and managed to work it out easily!) however it is coming from the back of the Mazda at anything above 15mph now! I changed the differential oil a few weeks ago and unfortunately it made no difference - if anything it was slightly worse The cool thing is you don't notice it with the roof down - but I know that's not the solution! Probably just to do with hearing noise outside more.

The garage had a look at it whilst it was in for some body work, however they are unsure too. The only thing they suggested is to disconnect the drive shafts and tow it! Is there any other way to work out what it might be rather than fork out more cash? Just spent a load of dosh on body work which was way over due, and it needs a full exhaust, cam belt etc!

I did spot a bargain differential (£25) on ebay but unfortunately it has now sold. New they are £440 and second hand units are around £120 plus £120 fititng! I guess I might have to take a gamble and go for the wheel bearings next - £30 a side plus labour.

Any help suggestions etc are very welcome! Many thanks