NannyBeth - it's up to 2 hours in a day... a day being 2am to 6pm (as 6pm to 2am is unregulated).
By wanting a higher salary for the hours whilst at the child's own home, if that was discovered then the regulator could try to use that to prove that you were really acting as a childminder.
However, in the situation you describe... how would the regulator know? It would really take someone to tell them.
I've suddenly thought that I could do from 9am until 3pm at her house (so as a nanny/babysitter)
Yes, those hours could be done as a nanny - so having the mum as your employer.
hours before and after, would have to be looking after the child in my house as I have the children to take to school (he would come with me) and then the pick up at half three.
Why would that have to be at your home... could your children not go to the other house?
What about the mornings... a benefit to the parent of having a nanny is that they don't need to get their child up and dressed in the mornings. They wouldn't get that benefit with you, if they need to bring him to your home.
I could then take him back with me to my house with my own children until they collect him after work.
To get around the 2 hour rule... you could just not go back to your house for a while... such as by going to the park, going to the libary, going swimming etc.
This would only be for 2-3 days a week.
Irrelevant to the question being asked.
Can I do that without being a registered childminder?
There are ways around it as said above... but if child is at your home for more than 2 hours, then it may breach Childcare Act 2006 depending on how the payment is viewed.
could I do it if she didn't pay me for those hours when he's at my home?
Yes. But any form of financial payment would be counted, so...
(obviously, she could pay me more the hours when I'm at her house to cover the other hours).
That's where it gets very grey area... as that is financial payment. Though could it be proved that the amount for the other hours has been increased to cover the non-paid hours.
That kind of sounds dodgy
So therefore you already know that it is probably doggy
but I know she knows she can trust me to look after him
But can she trust other people at your home? Are there any other adults living there, or who have frequent access?
and he likes me and I'm a mummy (which was top of her wish list before)
Not sure what being a mummy has to do with it... I'm not a mummy yet I'm great with young children 
As Fairimum says, insurance could well be an issue. Nanny insurance covers you for outings but would not normally cover you for taking the children to your own home on a regular basis - contact insurer to confirm what they will and won't cover. Also your household insurance may not be valid if you are using the property as though you were childminding.