Thanks for all your help, looks like I finally found what the problem was...
And you will NEVER in a million years think is was this....
The seal on the radiator drain plug needed replacing, all for the cost of £2.78 GBP for a new drain plug as BMW don't sell the seals separately.
Found this problem after I went to a BMW garage and they performed a Pressure Test on the cooling system.
The funny thing was that the new radiator I had purchased, only had 1 of the 2 seals on the drain plug that it came with. So it had the same fault.
Hope this helps anyone who may have a similar problem.
All the best, Jay