Congratulations on your high marks Shelagh! You worked hard for them so now you have your own prize ^^
On the mediums: I mainly use water and glazing medium (vallejo). Sometimes I use lahmiam medium too.
The difference I found between water and glazing medium is that the glaze medium elongates the drying time of the mix, it let's me move pigment on the surface of the miniature for longer periods and enables me to wet blend (if that is the word) better. On the other hand it sometimes makes bubbles on the colour mix (not that I am saying that it fizzles, it's just only that more bubbles than normal spring out when you are mixing the colours) and sometimes if you do not wait for the painted layer to dry enough you just "erase" it with the following layer. Ratios I use are: a drop of glazing medium and between two to more "brushes" of water depending of what I am doing and the colour I am mixing. On bigger mixes I put 2 drops of the medium.
Lahmiam medium serves (for me) as a glazing/wash maker for any GW colour you have, just play with it until you find it works as you want. I found that sometimes it leaves weird matt parchments, but that could more than possibly be my fault. Take a look at this vid if it helps:
https://www.youtube.com/watch?v=tSxrMa4OZXo as it also explains the difference between a wash and a glaze (I think you already know, but he? explains it very nicely.) skip to minute 5:40 aprox if you want to skip to directly how the ratios should be for each result.