Interesting. That breaker panel has no master disconnect. For that size panel older typically a 100 AMP breaker, newer typically 200 AMP. I've seen that once before and I guess it is (or was) code legal.
it's at the bottom ?
You're right. The primary is coming in on the bottom. It appears there is a master disconnect located there. Nevermind.
The odd thing here is that all the breakers are at the top of the board. Surely it is best practise to start populating the board at the bottom with the heaviest loads nearest to the incoming supply and the lightest load above them?
As heavy as the busses are, I don't think it's really necessary. The only thing I'd try to keep as close as possible to the main breaker would be a panel-sized MOV for lightning protection.
And as for Mike's comment about there initially appearing to be no main breaker in the panel, there actually could be instances in the US at least of the main panel not containing the main breaker - if the panel is located more than about ten feet (can't recall exactly) from the service entrance (meaning that more than ten feet of the service feed is inside the building before reaching the panel) then the NEC requires the main disconnect to be located at the service entrance rather than in the panel to prevent a long run of effectively unprotected wiring within the building, and the panel is treated as a sub panel with the ground and neutral separated from one another and joined at the meter box rather than in the panel.
I ran into this when rewiring my house as my service entrance is about 25 feet from the breaker panel, with the feed lines running through a crawlspace. I had to buy a special meter box that includes a main breaker (and of course the utility only approves certain ones, the most readily available of which set me back about $700 - highway robbery!!). It was great fun pulling three 2/0 and one 1/0 cables through twenty-some-odd feet of conduit by myself. Laid them out across the yard, taped 'em together and fed all four at once, and mostly pushed them through from outside. I used a standard main panel with a breaker in it, too, so I can still kill everything from inside the house, but if a fault should develop in the crawl space the breaker in the meter box will trip to protect things.
-Pat