if ALL the downstairs lights are flickering it is probably the MCB, most MCB's in household distribution boards are cheap and nasty and can easily go faulty.
a new one will roughly be about £8-£10, if you are careful and make sure you turn off the main switch, they are extremely easy to change.
if you decide to do it yourself the best wholesaler is Newey & Eyre as they can usually get you any make that they don't have on stock on next day delivery.
(Be cheeky and ask for discount if it seems to expensive)
the only things they will normally want to know is the make, rating (eg. 6 Amp) and type,
if the board is fairly new it will be a type B, if it is over 10 years old it will probably be a type 2 in which case just ask for a type B as you can't fit or buy type 2 MCB's any more.
if it is a Merlin Gerin(the best you can buy), or a MK I have some that will fit that you can have, they will also fit other boards but I don't recommend it unless they can be fitted without modification of the board
Thanks for the response guys, I think were it outside the consumer unit and i had a problem i would have a go but i think i'm going to get someone in to check just so i'm not chasing red herrings. I'm leaning towards the bad mcb problem as even with the mains off it won't flick on.