Oil Control ring

Oil control rings I take it is the oil rings on the pistons.
If so, then you’d be replacing the entire lot, oil rings/compression rings and having it honed in the process......
It’s not the type of job that someone ‘calls out’ to do, it’s major surgery !
It’s best having this done as part of a very comprehensive rebuild and recon and other parts will be required in the rebuild process. It’s labor intensive and relatively expensive.

What Jon said....

I wouldn't let this " Chap " near it if it were mine....Not a job you do on the drive way

yep, smoke is only bad on start up, the rattle is on the power strokes, but she isn't missing a beat other than that.

What JJF says is correct ,you are doing a rebuild and the bore should be checked to see if it is oval ,if it is too much, it is then a waste of time and money. If it is O.K. then remember to make sure that the ring gaps are on opposite sides otherwise you will get blow past and use oil

Does it have a turbo? could be seals in the turbo.

Either way I'd keep buying oil. It's a lot cheaper than an engine rebuild.

Killiand, those older 2.5 are old school technology, mine is the turbo version.
There is a few issues with them, but all in all they are a sturdy old lump.
They will Smoke on startup and run uneven if the pump timing is retarded, even by a small amount.... however retarded pump timing won’t cause a ‘diesel knock’ like you explain, but too advanced pump timing will.
Need to establish if it’s a ‘diesel knock’ or a ‘mechanical knock’ you’re experiencing. A mechanical knock in time with the pistons like you say can be crankshaft bearing wear or wee end bearing wear.... not likely with those if the oil has been changed frequently.
Diesel knock can be, like I said timing on the pump, leaking injectors (will cause smoke) or governor issues in the injector pump.
It’s a 12 valve engine, if oil had infrequent changes the likelihood is that the cam lobes will be worn, this has the result of hydraulic lifters being extended and causing a rattle which will increase with engine rpm... also will lead to power loss and retarded valve timing.
Smoke can be from worn valve stem seals like previously mentioned but you can establish yourself if it is using oil, how much and over what miles/hours use.
When I got my van the previous owner was a Diy’er who caused more trouble than good.....
I rebuilt it and replaced, Rings and honed cylinders (little wear), rebuilt the head and fitted new camshaft and followers, replaced white metal bearing although they were OK, fitted new oil pump and drive chain, had injectors redone and injector pump benched and set, fitted new coolant radiator and heater matrix, new water pump and new timing gear, had turbo updated and boost control increased to 1.7 bar.... It now starts perfectly, no smoke, uses no oil nor water, pulls like a train with the turbo upgrade and I’d trust it to drive it to hell and back, no stupid electronics to worry about and if looked after will run for ever.
Have yours looked at and get a report on its condition and take it from there, all parts are available and I got mine from a dedicated supplier in Lithuania via the Internet.
If the van is good then a proper rebuild is worth it.

Perhaps ignorance is bliss, but I have a strong faith that if I can get a hold of him, it'll all be sorted.. also the camshaft is in perfect nick!

Haven't had himself back out, tough man to get a hold of! Was on my to do list this week to follow up, but will be next week now!

The aim is a week in September, 2.5 in October and maybe one in vlnovember but they're not dependent on the control ring

I'd like to say my lips are sealed.... But they're far from that, just hoping to have news soon