I don't see how it can be a retention as such for the whole job - this is a sum of money kept back after a job is completed (often for about 6 months) to cover against defects occurring. In my experience (admittedly quite a long time ago now) any retention was usually around 2.5 to 5% of the contract value & was set out in the contract documents.
You are no longer doing the job so have no responsibility for the quality of the work, timescale or anything else - other than in relation to the work you did before you left the job.
If you had a contract, what did it say? When you left the job was there any outstanding issue about the work you had done? What % of the cost of the work you did is the £8K? All these are questions which may be asked if you took action against him - eg by issuing a Court claim.
It may be worth you getting advice from a solicitor - who, if he/she agrees the money should be paid now, could write a letter which might produce what you want.