More people will answer your question if you post it on the TECH FORUM portion of this site.

In the UK what is MB's warranty, two years? more?

Does your service manual have a recommended replace or check interval for the timing chain?

If it is a chain, in most cars somewhere between 100 & 150K miles.

If it uses a belt and not a chain, 60K is possible.

Had the car been given regular oil changes and service?

Whatever, four years and 70K miles is too soon. Can you shop dealers for a price compromise?