You will need to ask the Clamping company for a copy of their complaints procedure, and then go through the process of making an appeal on the grounds that you did not see the signs. This will inevitably fail. and you will then need to send a Letter Before Action giving them a period in which to give your money back or you will issue Court proceedings. Send everything by recorded delivery and keep meticulous records as in the event of issuing a claim you will need to demonstrate that you have exhausted all other avenues. Have a look
here for an outline of what you would need to do to proceed.
Perky quotes Arthur vs Anker as absolute justification for the clamper's actions, but that you would have grounds to argue that the fee was unreasonable. Whilst this is true, for that case ruled that the release fee must be 'reasonable', it also ruled that arrangements should be in place for the prompt release of the vehicle once you had agreed to pay. Depriving you of the use of your car until the following day would in most circumstances be contrary to this ruling.
A far better defence would be the case of Vine vs London Borough of Waltham Forest, where the Appeal Court ruled that the person clamping a vehicle must prove that the driver consented to the wheel clamping through not only seeing warning signs but fully accepting the risk of parking there. From what you have said, you did not accept the risk as it was not explained adequately to you - position of signs, dark street etc. Read up on the relevant case history and make an assessment of how far you are willing to go to recover your money.