Made sense of it now.
The manager, it transpires, had no intention of offering me the windows at that price, he was simply convinced that the salesperson hadn't done a good enough job of convincing us that the product was good. He wanted to ask us whether we would buy the windows if they were 500 quid; whether that was to test whether the salesman or try to get us haggling and signing, I have no idea. But he was adamant that he wasn't offering the job at that price!
The salesman had done a good job, but we told him from the outset that we wouldn't be signing today. And thats something the manager just doesn't understand.
In my opinion, having gone into the branch to discuss the offer, the manager is a pushy, over-confident, loudmouthed, hard selling bully. Was adamant that we wouldn't get the same price in a weeks time if we decide then, the price would be higher. Wouldn't even say that they'd re-assess and work out the price afresh, it would be higher. Which is, of course, a rather desperate lie.
Well, he gan go **** himself with a rusty fish slice. His product isn't demonstrably better or cheaper than anyone elses.
How on earth does someone like that ever sell anything?