SWALEC says deregulation means they are cheapest - Advice needed!

Seakay

I have just started to swap from Scottish Power (capped to Sept 2008) to E-on (capped to October 2009) and am a couple of days into the 7 day cooling off period.
A chap from SWALEC just called to the door and said that because they are the local provider they are cheaper than anyone on a comparison site because the sites quote national and not local prices.
I am very confused.
If the local provider is always cheapest then why do we do the comparison site thing? Or am I being very gullible?

SamDude

When you enter your post code in the comparison site, it uses your 'local' prices to compare.

Seakay

Well I though that but he said that they didn't, they only used the national prices on comparison sites and that post codes were only requested for staistical analysis.

SwanJon

As a rule, don't trust what someone says on your doorstep.
SSE (Swalec) have different electricity prices for different parts of the country like everyone else.
All those people who havn't changed are still with the 'local' supplier, and so there is less incentive for the supplier to try an impress. They end up spending their money in other pparts of the country to attract new customers.

Seakay

So he was telling the truth about SWALEC being cheaper here in Cardiff then elsewhere, but was he right in saying that the reason the SWALEC didn't look the best option of the comparison site is because only national prices are shown? Because that would seem to mean that there is no point in using comparison sites at all, just go for the cheap local option.
I've changed loads of times, but never had this sort of thing happen before - my instinct says "shyster" but I don't want to rule out a good deal just because I don't understand; he also said that SWALEC wouldn't be going up until next April and that they would still be below the capped price that I am going for after that because the local supplier has to have the cheapest price by law.
I have no idea how to verify any of this.
Any thoughts?