"Engine flush" is only useful if the oil has been left in too long and sludge has built up. If this has been left too long the flushing agent can cause damage as a load of carbon gets liberated and can cause seals to fail and the filter to get blocked.
All the reasons for not doing it on a 1.8 T apply to a diesel only more so. Boost is higher and so is compression.
If you have a basket case that has not seen a change for 40k miles - and as a last drastic step - by all means give it a go.
If it was mine -not at all. Ever.
Doubly so for a turbo engine - turbo bearings probably have the highest lubrication requirements of all the bearings in the engine. You are proposing to add something to the oil the designers never planned for and can only reduce the lubricating properties of your oil.
Give it an oil and filter change every 3k miles for the next 12 months. Fully synthetic has the best cleaning/suspension agents and out of those one approved for extended drain will have the best. That will do all the flushing you need gently and in a controlled manner.
Once that is done - stick to the service schedule (or even better more frequently) and use a good quality oil and you will never need a flush. The oil and filter will do the job they are designed for.
That is to keep the crud from your engine in suspension and filtered out until oil change time when it gets removed from the engine.
