Multiple Rated Areas Sharing Same Wildcard Zip/postal Code

Hi all,
I am struggling to find a straight answer and hope someone can help please?
Our store has a UK Mainland shipping method set up with one set of prices covering all the UK mainland using a list of postal codes with wildcards etc. This method is limited to 99.99kg and works fine.
I would like to add a new delivery method that caters for orders weighing 100kg+. I have created this method with weight of 100kg - 500kg.
The issue I have is each zip / postal code needs to be priced differently for this new shipping method (100kg - 500kg).
So to test I have created a rated area just using ST* as the zip postal code, nothing else. I have then set a price for the new ST* rated area in the new delivery method 100kg - 500kg.
Now when I calculate shipping using that postal code I get the "shipping unavailable message" no matter what weight of items are in the cart. All other zip postal codes calculate as expected.
Can anyone confirm if this is because a wildcard in the zip postal code section cannot be repeated to more than one rated area?
Is this the case even though the 2 shipping methods that refer to the ST* zip postal codes in the rated areas have different weight restrictions and therefore should not overlap with one another?
I hope that makes sense!
Thank you in advance!