Usual trick here is to act as the "stupid customer"don't forget to tell them you're in First Class before ordering (I didn't - and got a proper telling-off for it)
If you know what you're entitled to and you just want a cup of tea though - you need to be sure to ask for a First Class cup of tea (the free tea is different stock or something).
Thanks all - will try my luck with the "dim customer" approach. If there's anything more than free Coffee/Tea I'll let you know!