My apologies - your description was so similar to the two other questions I assumed you were connected to Brian Scott and Dopey who both mention earth wires and the same colour combination at the ceiling.
I would still suggest a sparky because - It's a bathroom - There's no Earth - you have seven wires which I can't quite fathom. Power cables always have Black Red and Earth and you seem to be missing one Red and several Earths!
If the Three Reds were originally twisted together - this is the usual arrangement - Live in - Live Out and Live to the Switch. But three blacks twisted together is odd - I would suspect it to be just two black wires - Neutral in and Neutral out. I strongly suspect that the separate black wire is indeed Switched Live. You have a meter...
Have the black wires hanging loose and apply the power with the three reds connected - operate the switch and see if the lonely black is actually switched live - If it is - it turns the light on and off and should connect to the Live Terminal of the fitting.
You do not connect any of the Live wires to the light - just the switched live - these three reds together neeed their own connection block somehow somewhere and it would be easy for you to make an unsafe installation - bad enough with Earth protection but with no Earthing potentially lethal and very naughty in a bathroom.
The neutrals connect together and also to the Neutral of the light fitting. A 4 way terminal box tucked into the ceiling allows for Earth, Neutral, Live and Switched Live - you could run a short cable to the light fitting from the terminal box.
I think you have been "stiched up" by a previous poor installation - If I were in your situation I would bodge around and make it work but still call a sparky to make it safe as soon as I could - I am not advocating that you do as I would.
A light fitting with an Earth Connection really does need to have a decent earth for the safety of the people using the room. TAKE CARE
Peccavi
March 2009