Craig
Your hourly rate should what you need to earn each working hour for you to live on and for your business to make a profit and be able to grow.
You should have done a business plan when you first started, to find this figure, what you need to do is list everything that you will spending money on this year including an allowance to cover unexpected things happening. Add all of this together and don't forget the Inland Revenue slice of cake, from this figure you will find out what you operating costs are per working hour, don't forget to make allowances for when you can't work though sickness, holiday and weather.
Then you need to add a profit to this figure, as for charging customers more for 1st cleans, this is nessasary because 1st clleans take more time and time is money to you.