There are thing you can do before hiring private detectives, many law firms hire them and I'm sure they could have advised the Fund on how to find reputable ones;
Jo Pizzala;
‘I would ask if they were ISO-registered, had Edexcel accreditation, had undertaken Skills for Security training and had passed their Criminal Records Bureau checks. I would want confirmation that they were solvent and their staff were experienced at giving evidence in court. Do they have fraud capabilities and undertake corporate investigations? Do members of staff have ongoing training in the latest technologies and techniques? For that matter, does the firm regularly update its equipment?’
your best option is to hire an ABI member,’ Robertson replies. ‘The Law Society of England and Wales, and the Law Society of Scotland have both endorsed the ABI.’
Gavin Robertson, a former Metropolitan Police detective superintendent and now a member of the Association of British Investigators (ABI);
ABI membership is a one-stop shop,’ says Robertson. ‘To become a member they have to undergo a criminal records bureau and financial probity check. They have to pass an entrance examination to prove their competence and have professional indemnity insurance. They are also regulated by the ABI code of practice and disciplinary code, which in turn are independently adjudicated by a solicitor appointed for the task.’
http://www.lawgazette.co.uk/practice/how-to-hire-a-private-investigator/5040588.fullarticle