Thanks for suggestions - Neither acetone nor petrol have any effect on
his foam - there is a commercially produced Foam Eater, but it's a
thick paste and labelled Do Not Use On Aluminium so thats no use. Heat
has interesting possibilities though - I'll see how the foam reacts to
it. Maybe I'll try taping a soldering iron to a long batten!

More suggestions welcome!

Cheerrs,
David.