wanna get the missus a cartier watch for our wedding anniversary

Just ask the seller outright how much he can do it for. Try half a dozen different jewellers and negotiate. They bare ripping you off at full price.
Why not check for prices outside of London...sure to be cheaper.

Trotters Independent Traders or A Daley esq will do you a good deal

Does it need to be real?

One of those tricky situations when I was faced with a similar problem last year re a ring I wanted to buy my wife.I ended up asking her to chose a ring in a jewellers she always looked into the window of but was quite expensive. But it was closing down and they had a sale.
One option might be to go to a high end pawn broker who want to flip things they buy and due to buying them at a big discount in the first place wont try and put the hand into you over price.
Sometimes a surprise present is nicer but if its something your wife is going to wear every day it might be better to let her make the choice and make her feel special going into a jewellers are having the staff pamper a bit by telling her how great her husband is and how lucky she is to be treated to such a nice (expensive) gift.
Got my wife a wee keepsake in a jewellers last week as a thank you and it cost a tenner and she loved it.

Sorry missed the bit that it must be brand new. I personally might use a pawn shop if I was looking for something older you would not get in a new shop but that's just because of my wifes personal taste in some things.

Pick ONE specific Cartier model. Shop around for comparative prices from various jewellers for that particular model. That should give you some idea of where or who to buy the one that your wife likes.
Are you worried about trying to get a few quid knocked off their price? If so, what is this thread about?
Why not treat her to something else with the money you save? That way she can't complain about you being tight...if that's what you're worried about.

On the one hand it might be better to take your wife to chose which one she likes. On the other hand she might not like to chose one which is too expensive. Bringing your wife along also limits your haggling abilities. You don't want to look cheap.

Here is one tip. Bring your wife with you and if its within your budget tell her to forget about the prices and ask her which one she loves most. Don't make it obvious to her your going to buy it and then return to the shop alone and try and get a better deal if you want to save some money. Then surprise her with a little box to open over a romantic meal in a nice restaurant. She will even think it cost more than it did and that you wanted something she liked best and wanted to make it a surprise at the same time.

Plenty on e-bay,cant go wrong there.