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Just a thought.

I bought an airbrush (infinity 2in1 - beautiful instrument) and some Golden acrylic medium for the airbrush, so I could thin the GW paints I have to use in it.

This was fairly unsuccessful, the paints still regularly blocked the airbrush or had to be mixed to the extent that the paint 'spiderwebbed' in big blotches. The mysterious 'sweet spot' evaded me and was frankly massively frustrating to try and achieve.

Would taking a simple window cleaner serve me better? I used windolene back in the UK in a simple airbrush to good effect. Can anyone recommend a better method?

Vallejo Airbrush Thinner will works for me.

Also, what needle size and PSI are you using? Those will factor in how you thin as well.

Using the larger of the two needles the infinity comes with, with an iwata smartjet compressor.
A 0.4mm I believe according to the nozzle sizes listed with the brush.
http://www.chicagoairbrushsupply.com/in2in1aiwiiw.html

Also, running the pressure at 25psi.

Definatly stick to the larger needle and tip for home thinned acrylics untill your feeling good about thinning
I use a H&S Evolution Silverline 0.2mm and I love the thing, the infinity can only be an improvement!

I use Liquitex Airbrush medium and sometimes a spot of water, but try to avoid the water becasue it breaks down the surface tension of the paint too much and causes spidering and really long dry times.
Try a glass cleaner, but avoid ones with vinegar in... an automotive glass cleaner should be suitable. This is because vinegar and a few other choice chemicals corrode chrome rather well. Considering you paid a whole bunch to have triple plated chrome inners on your nice airbrush, I would encourage you to be aware of what goes into it!

Edit; I forgot to mention that water based acrylics on the whole all suffer from tip dry and blocking with airbrushing. its just something that you learn to deal with if you have to use those kinds of paint. because of the sheer amount of air rushing through the brush passed the needle it is inevitable that paint will dry up around it and cause problems / clogs. A nozzle reamer might be a very useful tool for justa few bucks, I'm sure H&S make one that matches the contour of the nozzles they use perfectly.

I gave up trying to thin the newer GW paints, for what it cost me in wasted paint I decided to buy Vallejo Model Air paint. It comes in dropper bottles and is absolutely fantastic!

I use Tamiya thinner with my airbrush and it works fine.

Saying that, I haven't tried the new range with the airbrush so I don't know how well they work.

The old ones were fine, they were really good to airbrush I found. I just can't get the mixture right like the OP said, especially with the bases/layers etc

If anyone finds a good ratio I'll be glad to know, also. I'm using testors thinner coz I can't get my hands on anything else just yet

Not to sound like a broken record here (because this has been talked about a lot around here this past week), but there is no "good ratio" for any particular brand of paint. You can't make a blanket statement like "all brand X paints need a drop and a half of thinner to be perfect" and expect that to work every time. It is one of the most important skills that anyone needs to learn when beginning to airbrush and unfortunately there is no easy or "correct" answer. Grab some paint, your thinner of choice, and a piece of paper and go to town. It is the only way to truly learn.