Hi Nick, I would use a product called Force by Premiere Products (probably available through your local Jani distributor @ circa £8-10 / 5 litres)
Scrape off all the thick deposits, apply Force as per directions, allow dwell time then either rotary scrub and/ or hot pressure wash off.
NB You will have to be really carefull with the run-off if your drains don't have an interceptor. If they don't, block them off then wet vac up the slurry and allow it to settle in a 45 gallon drum and skim off the oil for proper disposal.
If the blue drums in the pic are full of the stuff you have a potentially big waste disposal cost there so don't give your customer a fixed disposal cost!