You can't have sticking injectors on a mechanical injection diesel - they're not like petrol inejctors where they are opened and closed with a constant feed pressure - they only open when the injection pump raises the pressure sufficiently, then they close again when the injection pump cuts the pressure. They're bullshitting you.
I think it's time to get your hands dirty and get the top covers off and have a look at the belt timing. You might need to take the glow plugs out and see if anything comes out if you try and turn it over (by hand not on the starter!).
TBH though I think they've buggered it up beyond redemption now. Probably going to need a replacement lump, then issue a small claims summons
Pity you're not nearer, I'd come round and have a look and write you an engineer's report, and give you an invoice for several 00 £ so you had some grounds to go on.
