But did they "fob you off"?
Assuming you're talking about who you appear to be talking about, their Terms and Conditions are clearly available on their website, and state that going to the manufacturer "is often the best solution", and also that this does not affect your statutory rights. They are correct on both points.
They then outline the situation if you opt for a Distance Selling Regs return (para 3) and it seems to me to be a pretty fair summation of the rights you have. The only point I would take exception to is that IF the goods can be rejected because of another contract clause, including an implied clause embedded in the contract by, say, the Sale of Goods Act, such as the goods being faulty, then they are not entitled to expect you to pay the return shipping cost.
Obviously, we're not privy to any direct communications between you and the supplier, but they're quite within their rights to suggest that you'd be best advised to deal direct with the manufacturer IF you want the monitor replaced. BenQ may well do it straight away. But if you return goods to a retailer, you do it either under the DSR (in which case replacement is not an option), or you do it under the SoGA, in which case you're entitlement to repair, replacement or refund is contingent on it not meeting the SoGA requirements, which usually means it's either faulty or not fit for purpose. And in that case, they're perfectly entitled to check the goods out, and decline to do anything about it if they aren't actually faulty.
Retailers, for instance, are NOT liable for problems that occur because of compatibility with your hardware or software, UNLESS you made that requirement clear before purchase. It would be entirely impractical for any computer retailer to be held to that standard, because of the vast range of permutations of hardware and software out there.
I'd bet that any retailer will have a lengthy list of situations where goods have been returned as "faulty" when in fact they work perfectly, and the problem is either to do with some other component or software in the customer's system, or even faulty installation by the user. I'm not suggesting that applies to you, but retailers can't tell how technically savvy or competent their customers are, and there's some right numpties out there.
So IF you return goods under the SoGA and it turns out they're not actually faulty, you're likely to end up paying both a service charge and for the shipping both ways. If, on the other hand, you get BenQ to come to your door, you avoid that.
It obviously depends on exactly what you were told by the retailer (which is a level of detail I'd rather not get into), and it's possible that an individual in CS didn't word a reply very well, but the retailer I assume you're referring to makes the general situation pretty clear on their website and, in many cases, will be right in suggesting that going to the manufacturer is the best option, because it often will be. That, in itself, isn't fobbing you off. If, on the other hand, they refused to deal with you at all and insisted that the manufacturer was your ONLY option, then that's when Trading Standards will likely explain the reality of the legislation to them. Given the pretty clear explanation on their website (far, FAR clearer than some retailers, I have to say), I'd suggest that if they did refuse point blank to deal with it, that it'll be a mistake by an individual not company policy.