Negotiating advice please

[TarquinMX5]

I'm thinking of px'ing my MX5 Z-Sport, which is a sought-after limited edition, for a new MX5, one with the newly announced more powerful engine.

I'm after some advice from people who've successfully managed to get the best deal possible from a salesman (or saleswoman or whatever).

I heard on the radio this afternoon a view expressed by a Mr JC (no, not that Clarkson chap, or the other JC, but a Mr J Corbinn) who was stating that the best way is to 'take no deal off the table'.

I've never approached negotiations thay way before but I'm just asking the experts on here, would I be likely to get a great deal on my new MX5 if I tell the salesman, upfront, 'look, let's be quite clear, I'm not leaving the showroom without a deal. I must have the MX5 and I must have a deal'.

I've seen an advisor advertised, a Ms D'Abbott, who seems to work miracles with figures, and who has said it shouldn't cost more than about £250 for a new MX5. Does that sound possible, or is that some funny form of sums?

What are my chances of clinching the deal of the century?