Thanks everyone for your input.

I spoke to a lady at the US embassy in Dublin and she confirmed what most of you have said. It is entirely up to the immigration officer if you are allowed enter and how long you can stay. It is not 6 months in any 12 but a maximum of 6 months at any one time. You can leave and apply for re entry anytime. This is just what I needed as I do not want to work there but would like to come and go for extended holidays.I am astounded at the wait times in London. We got appointments for three days after we applied. We got our B1/B2 visas in Dublin and it took all of 15 minutes from entering the embassy, interviews (my partner had a seperate interview) and being granted our visas. My interview lasted all of two minutes and my partner's lasted 30 seconds. The immigration officer said he would interview us seperately so we could take turns looking after our 2 and 3 years olds. We received our 10 year visas in the post next morning.