I think that solicitors tend to charge for the District court in advance where it's an unknown client and it's a one off. Say public order offences, motoring issues etc. The solicitor knows you but you are not a client, which is key to why he wants the fee up front. I presume you've picked him because he's good, you've agreed a fee, so I'd just pay it. He doesn't want to be chasing people for money afterwards, it's different if your a client as he would know that you would for sure pay him (in general).