Hi dot1. I'm really sorry you've had a disappointment about the clomid funding, and I can understand that it seems unfair. Presumably you can still have the treatment if you go private? Clomid treatment is not that expensive (the scans are the main cost).
Some information: the eligibility for NHS funded fertility treatment is determined locally by the Primary Care Trust (formerly the local Health Authority - these were replaced by primary care trusts in April). The precise rules vary greatly from place to place. In many places there is virtually no NHS provision at all, in other places it is relatively generous.
However,I don't think any area provides NHS funded fertility treatment for people who already have a previous child. Now of course in your situation it is the definition of 'previous child' which is the key. It may (or may not) make you feel better to know that in my area, I'm pretty sure that the definition includes any child to either partner who is living with them. So for example if DH had a child who was living with us, I would not have been eligible for any NHS treatment, even though I had not actually had a child of my own.
I suggest that you contact your local Primary Care Trust and ask for a copy of their eligibility criteria for NHS fertility treatment. Then you can see whether it looks like you have grounds to request NHS funding - it may well be that they haven't had to consider people in your situation before, and that may give you the opportunity to put your case forward to be considered as any other childless woman, rather than as the partner to someone who already has a child (IYSWIM). Its a grey area and therefore worth arguing the case - but you need to know the details of the 'rules' in your area.
I'd also discuss this with your GP (who you've said is sympathetic) and tell her that you'd like to read the eligibility criteria and possibly challenge the decision that you are not eligible. She is probably just trying to implement the criteria as she understands them.
There are many many ways in which access to fertility treatments is unfair, and you've fallen foul of one of them. Hope this gives you some information to try and do something about it!