No doubt they'd need something more concrete but it's also worth noting that after Everton complained so vigorously about Clattenburg's refereeing in the 2007 Merseyside derby at Goodison, they succeeded in getting him banned from reffing any Everton games for a full 7 years afterwards. Now I'm not sure Everton were in any possession of any evidence pointing to Clattenburg being biased against them (although some of his decisions were dodgy as fuck in that match) but by complaining they got him removed from any of their games for a very lengthy period of time. And while I say fair play to them for succeeding in that, it's unfair on all the other clubs who had to put up with Clattenburg reffing their games during that 7 year spell.
Oh, and fast forward a couple of years from that 2007 derby debacle to one of Hughes's last matches in charge of City before he was sacked - a 3-3 draw at Bolton. That was a match reffed by Clattenburg too and he wrongly gave Bellamy a second yellow card for diving when any fucker at that end of the pitch could clearly see that he was fouled. What followed after that match were a series of revelations by City's management staff about comments made by Clattenburg around that game, which if true would've presented a clear case of him being biased against us when reffing that match. Much of that story has now been largely forgotten - not least because Hughes was sacked a week later and that became a much bigger story - but I remember hearing about it at the time and thinking if it had substance then there's no way that prick should be anywhere near a football field.