Hi Phil, I was helping David at DB Diesels at the mechanex show at Harrogate last week were I met Michael McDowell (nice to meet you). I also have an interest in injector back leakage and I have been stripping injectors and fitting seal kits most of this weekend it’s easy for me because I have the tooling and access to test equipment to check my results.
You need to remove the injector because it cannot be done in situ. You will then need to replace the seals which are not expensive (about £17.00 from DB Diesels which will service 10 injectors) but you may also need to remake the seat of the ball. This is the part that allows some back leakage to return to tank when the solenoid is operated. It also has to seal when there is no injection or the back leakage will continue to flow.

D B Diesels also do a tool to repair this. It operates by connecting to your workshop air-line and is not expensive but you will need to contact them at this address info@dbdiesels.co.uk

Also if you have problems removing the injector you can rent a hydraulic tool from them that will do the job.
There is no reason back leakage problems can’t be sorted out in house with some investment in tools and some know how. If anyone needs more information post your e mail address or e mail me at martin@wainwright-bros.co.uk and I will get back to you.
Best regards. Martin