I'm probably going to regret it but I've just spoken to Nova Chrome (as per Customprintwales comment that they had good results) and managed to order a sample set to try out rather than a full case of 36.
I figured trying just one more supplier might be worth a go before I give up too!
I'll report back on the results and if they're equally duff then I'll at least know to give up looking for alternatives and move on to something else!