I understand the PCD of a wheel ie:-- 5x100 means it's 5 stud and the distance between hole centers is 100 (I assume mm's) but I'm not quite understanding the ET

The smaller the number the further they stick out????

If the et of a wheel is 43 will it be ok to replace it with a wheel of 38 et.......

its very complicated, as I am finding out myself. For example a 7.5J wheel with ET46 sits 10mm inboard of an 8" wheel with an ET of 42, so if I were to change my allows from ET46 to ET42 then they will stick out another 1CM.

I found this quiet handy

http://www.1010tires.com/wheeloffsetcalculator.asp

Have a look at this brilliant guide written by pip_vrs

The smaller the number the further they stick out????

Yes

If the et of a wheel is 43 will it be ok to replace it with a wheel of 38 et.......

Assuming the wheel is the same width, it will push it 5mm further out, so as long as you have 5mm clearance under the arches it would be fine