Chain slipping

[StreetCowboy]

My road bike went in for service, and I picked it up this morning, with its new handlebars, bar tape, gear cables and more relevantly, chain and cassette. It’s an 11-32 in lieu of the 11-28, so I was worried about the small - small.  Some adjustment, and that was ok; the mechanic had warned me that the big chain wheel teeth were worn, and since the service the chain has started occasionally slipping under load - particularly big chain wheel to small cogs, no problem with small chain wheel.  
Based on prior experience, I asked the mechanic to tighten the rear derailleur tension, and I tinkered with the rear derailleur adjustment, which may or may not have improved, but not eliminated the problem.

Any suggestions for optimisation, while I await a new chain wheel?  I did not suffer chain slip before the service - problem may be new chain, old chain wheel

[frequentatore]

It happened to me a few times. A new chain may slip on old chainrings or cassette.

(I fix everything myself)

[StreetCowboy]

Thanks!

I had a look through my notes - this is my third chain since putting a new groupset on my bike, with approximate mileages

5,000 km

5,000 km

8,000 km, so I reckon a lot of the wear happened in the last couple of thousand km.

I suffered a similar problem when I replaced my chain on my old shopping bike after about 5,000 km - the shop did not have a cassette in stock - so I bought a cassette at Decathlon.  I guess I will just have to quarantine the big chain wheel until a replacement arrives.

[ChristianBlessing]

Is the chain worn or stretched? It sounds as though it may be.

[StreetCowboy]

55 minutes ago, ChristianBlessing said:

Is the chain worn or stretched? It sounds as though it may be.

The chain is brand new; the chain wheel has 18000 I’m on it.  When I checked my notes, my previous chain wheel lasted 19000 km before I upgraded my group set to hydraulic brakes.

I reckon a lot of the chain wheel wear happened in the last couple of thousand km; I was told this chain did not need replacing at 5000 km, but maybe I should’ve replaced it anyway, on the basis that it would definitely want replaced before 7500 km.