ANSWER 9th September 2010
Hi Dave,

This should be quite easy. A good broker should have the facility to deal with London Market Makers who make a market in the US stocks you are interested in, out of US hours (8.00-14.30). The same MMs will trade in the States between 14.30 and 16.30. It may get a bit difficult after London hours depending whether the London broker runs a skeleton staff in the evening.
If you are trading in London before US opens then the price will be adjusted by the MMs as to what they think will happen when the US market opens. Whatever happens you will probably be paying two lots of commission: one to your London broker and one for the US broker (which may be in the price.i.e. a net price).
The trade is usually done in dollars and the broker will have a facility for issuing you with a GBP contract. You will also , more than likely, need an authorised depository, again, which your broker should have.
The settlement is usually three days and you may not be able to have extended settlement.
If you would like to discuss this further, why not join our forum.
Don't forget you need to sign a W8BEN as a non-US citizen otherwise you will be liable to double taxation. Your broker will organise this.