Going back to the original point of my post: My lovely g/f tonight suggested this possible solution which I can't see much fault with but any other opinion would be appreciated:

She suggests she visit me for a month or two next year as we originally planned (on a fiance visa) having left the kids in the care of her mother (not ideal I know but!). Assuming we then get married here in UK she then goes back to Philly and lives with the kids well away from 'him' (Mindanao as opposed to Manila) and having broken all contact and monetary support, for 3 to 6 months or so at which point we apply for the kids Visas and proving sole responsibility shouldn't be an issue then. (There is court precedence http://www.bailii.org/uk/cases/UKIAT/2006/00049.html that the actual period over which sole responsibility is demonstrated has no specified minimum, so the fact that 6 months previously she wasn't solely responsible should not matter).

Sounds like it should work to me, unless anyone can suggest a particular problem?