Octavia ll 1.9 tdi starting issues

Hi all,

I've been having a problem with me Octavia 1.9 tdi since about July now. It started with a rough start but after turning it over for a few seconds it would fire up. Halfords replaced the battery which did nothing at all. Another mechanic took a look and thought that the fuel was draining back into the engine so installed a 1 way valve, this also didn't help. I decided to leave it for a while but it got progressively worse. I got some advice to prime it a couple of times before starting and this helped for a long time but then started to not have an effect. I then got told to park it on an incline (engine down) overnight and this again helped for a while. Now it doesn't matter how it's parked it just wont start in the morning. 

My friend recently took a look and claims that the fuel is black in the engine and he thinks that the injector seals are leaking oil into the fuel. I then took it to a larger garage that could do the job and they did not want to take the advice of another mechanic and plugged it into diagnostics and it came up with a glow plug fault. They replaced the glow plugs and all seemed good till 4 hours after picking it up, it was dead. I then took it back and they had it for 4 days and claimed it started every morning so they didn't investigate. I took it home yesterday and this morning it is dead. It now won't start after about 2 hours of being left. The glow plugs seem to have prolonged how long it can be left and has helped with starting a little but hasn't solved the problem. 

Any advice? Shall i go straight for the injector seals or could something else be at fault? Any advice greatly appreciated.