Why is my chain slipping/dropping on the smallest chainring when pressure is applied? Why is my chain slipping and dropping on the smallest chainring when heavy pressure is applied (hill starts mostly)? It seems to be worst when the chain is on largest two cassette cogs.


The bike just came back from a full service at a reputable LBS after
a collision with a car, this issue wasn't apparent after the
accident and appears to be a result of the recent changes/service. 
There is no (excessive) chainring wear. It looks as good as new.
Cassette, cables and chains are all new and fitted by the LBS and shifting is perfectly smooth.
The cassette was changed from 28-12 to 25-11. Chain is correct length and 10spd which is appropriate for the bike and is properly tensioned.
Rear and front mechs are both aligned and limited correctly, hanger alignment also fine.
No frame damage, the LBS assured me of that.


Am I missing something, I am completely baffled and frustrated as to what is causing this.
You say the cassette and chain have been replaced but the front rings have not. You also mention that the front rings look ok, which maybe so, however it seems there is still a meshing issue between the new chain and older front rings. Assuming the frame or bottom bracket are not misaligned. I would take it back to the bike shop and have them check the chainrings over fully. 
Your not just looking for worn teeth on the rings but the way the chain locates on it.. Even slightly worn teeth can cause issues with a new chain. 
Also check for a loose chain side crank arm or cracked/loose chainring bolt.