Nutritional breakdown for a 'Naked' GBK Wellington burger?

Hi.

Quite new to this but loving the motivation this app and my fit bit are giving me. :-)

Anyway, got some vouchers for GBK which makes it a cheap dinner for me and my gym buddy but they don't have any calorie / nutrient info on their website at all. Does anyone have any idea what it would be please?

£8.45
The Wellington
Grilled mushroom, horseradish sauce, rocket, pickled onion, truffle mayo, 6 oz Aberdeen Angus,beefburger.

I 'guesstimated' it at around 600 calories but it's too easy to put myself in denial.

Thanks.