So:
What am I allowed to do when I enter the US on an ESTA? Am I allowed to work if I am getting paid in a UK bank account? Obviously a potentially contentious issue and I'm keen to get it right!
The Visa-Waiver Program, or VWP (which is what the ESTA relates to) allows you to enter the US for the purposes of Tourism or Business, but not for 'work'.
The distinction here is really down to where you are paid. Presuming you are already working for this company, and being paid in the UK, then your visit to their US offices is classified as a "business" trip, and is thus eligible for entry under the VWP.
Whilst you are in the US you are able to participate in 'business' for your company, but in effect this must be the same type of business that you would have been carrying out whilst you were in the UK. They can not pay you any additional wage (other than expenses/etc) over what you would normally have done in the UK, and they can not pay you locally in US dollars or in any form that would require you to have a US Social Security Number.
ie, you're good! This is one of the purposes the VWP was designed for, and I've personally done exactly what you're describing at least a dozen times (from Australia rather than the UK, but otherwise the exact same situation)
The definition of what is allowed under a B-1 visa (and thus also under the VWP) is defined in the U.S. Department of State Foreign Affairs Manual Volume 9 - Visas
The relevant section is :
9 FAM 41.31 N8 ALIENS TRAVELING TO UNITED STATES TO ENGAGE IN COMMERCIAL TRANSACTIONS, NEGOTIATIONS, CONSULTATIONS, CONFERENCES, ETC.
(CT:VISA-701; 02-15-2005)
Aliens should be classified B-1 visitors for business, if otherwise eligible, if they are traveling to the United States to:
(1) Engage in commercial transactions, which do not involve gainful employment in the United States (such as a merchant who takes orders for goods manufactured abroad);
(2) Negotiate contracts;
(3) Consult with business associates;
(4) Litigate;
(5) Participate in scientific, educational, professional, or business conventions, conferences, or seminars; or (6) Undertake independent research.
It's also worth specifically reading the section in that document regarding the "Board of Immigration Appeals in Matter of Hira", which basically holds that partaking in work actives whilst in the US, whilst not being paid locally (other than expenses) as a part of future work that will principally be performed outside of the United States, IS covered by B-1 status. You can find a copy of the Hina ruling on the Department of Justice Website.
The main confusion with B-1/VWP is the very mis-used term "work". In general in immigration law, "work" when used in most contexts actually relates to being employed and/or remunerated (other than reasonable expenses) in that country, not the act of actually working.
As an example of this, the Department of Justice/INS document "PERMISSIBLE ACTIVITIES FOR B-1 BUSINESS VISITOR" specifically states that the "B-1 classification applies when a foreign employer requires an alien employee to work temporarily in the United States (US) pursuant to the foreign employer’s international transactions." ie, it is permissible to "work" in the US whilst under B-1 status, but not to be "employed" in the US.
(Note that the "foreign employer" part of this statement will almost certainly be true, even though the person claims to work for a "US Company" on the grounds that the actual employer will be a local subsidiary of the US company - at least from a legal perspective)
