Hi Lulubear, sorry to hear what you've had to go through, you must be feeling very frightened.
I had an ectopic about 2 years ago, and another suspected 6 months later - and from my experience they are not generally very good at diagnosing it. I was told mc, then threatened mc, then normal pg, then finally ectopic and surgery with the first - and given the really helpful (not) leaflet titled 'pregnancy of unknown locatoin'.
However, the fact that your hcg is dropping is really, really good - I think ectopics usually have rising hcg still.
Is your bleeding heavy? Mine was very watery and dark brown with the definate ectopic, and the pain was quite unusual too, more in the bowel area than stomach cramps.
Please do go back and ask them to keep monitoring your hcg if they aren't already doing this. They should monitor until it goes back to less than 5.
Ignore your gp, they obviously aren't going to be any help, sadly that seems very common too. Call the pregnancy assessment unit, they are the best people to help you, and ask for another scan and ask them what they think it is again - sometimes the more you ask they more they will tell you in the end.
I'm sorry this is a bit rambling, hopefully there is something useful in there! I do think the fact that your hcg is going down is very promising though, so hope all is well. By the way, if you are mc it doesn't mean that it wasn't ectopic, you can mc an ectopic as you would a normal pregnancy.