Following on from the above, my advice would be if you can afford to say no, then say no to any wedding party where you will not have direct contact with the end user. It's a recipe for disaster. If it's through an agency, may the Lord have mercy on you. In my experience, the attention to detail you can expect from an average agency is dire and wholly inappropriate for providing a decent service for what is arguably one of the most important events.
Some of my experiences and those of friends of mine:
1) Agency tells DJ that the function they want covered is in a particular room, which is an absolute mare to have a gig in anyway, within a large venue that feels like it has a mile hike from the lift to the actual room. Plus there is no on-site parking allocated so you often have to park in a multistory that's a good 5 minute walk away. DJ sets everything up early in the day as a consequence. DJ then arrives back in the evening to discover the agency goofed up and that the DJ was actually meant to be covering at a completely different venue across the other side of the city!
I kid you not, this happened. Thankfully not to me because I'd have probably resorted to maiming the person at the agency responsible.
2) Agency promises to obtain salient details such as name of bride and groom and first dance for you. You even try ringing venue to explain you're keen to ensure the event runs smoothly and that this information is of the utmost importance to have. The venue takes your details and advises they'll get someone to call you. They don't. You then have to deal with a hysterical bride on the day who can't believe you haven't got 'Dear Prudence' by The Beatles with you that night because that's their first dance. Why would they need to bring a copy of this "obvious" choice for a first dance?
A lot of us will have variations on that theme. Now, I should note that the above happened to me in 2000, so back in what would possibly be regarded as the last days of the equivalent Neogene period for mobile DJs where mp3s were yet to take off in any meaningful way. I'm a big fan of The Beatles, had the White Album at the time but this was an album I'd have never considered taking with me as I left home to go to an average wedding party.
3) Agency tells DJ the function is a 21st birthday party and they want lots of garage music (showing my age!) The reality is it's a wedding and the couple could give Henry VIII a run for his money because they're both getting married for the 4th time. Oh, and they have a combined age of around 110 and specifically wanted Trini Lopez 'If I Had a Hammer' played at some point.
4) Agency has for once, it appears, actually spoken to the bride and not just some oik at a venue. However, agency has not acted in the best way by telling the bride it will definitely be a particular tribute act that she saw a few months ago because they actually send someone else. In addition, the bride informs the DJ that she's paid the princely sum of £1,000+ for tribute and disco (DJ is getting just over one tenth of the total) and she specifically asked the agency to tell the DJ to NOT play a track that was incredibly common to be played at the time because it happened to be played at her brother's funeral a mere few months before the wedding. The track being 'Angels' by Robbie Williams. Did anyone actually pass this on? NOOOOOOO!
What happened there was another guest, who'd also been at the funeral, and felt it might be a nice touch to get that song they'd had played for departed relation on at some point. However, they had just come up to ask DJ 'can you play 'Angels?' without explaining its significance.