We cannot recommend any particular firm however an Internet search should identify any firms who work on these terms.
However, firms that operate on no win no fee basis will undertake an appraisal of the case to establish the likelihood of success. If you do not have a good percentage, I.e at least 75 - 80 % then a firm will not take it on.
An issue here will be where there is any written agreement between the parties. If not this will seriously undermine the case and changes of success.