I need to know how to do this, would appreciate some help.

The setup required is for the Mikrotik CPE that allows another ADSL router (connected via CPE) to be used as a gateway.
basically we need to setup the Mikrotik CPE so that it acts as a cpe plus a link for the adsl. so that the adsl can be used as a second gateway
i just made a quick sketch to show how it would work, please find it attached.

Do I understand correct?

You want the CPE to use your network as the primary connection and the ADSL on their premises as the backup/failover.

Other customers/CPEs should also use the ADSL at the highsite as the primary link and the one on the left of the image as the failover?

Do I understand correct?

You want the CPE to use your network as the primary connection and the ADSL on their premises as the backup/failover.

Other customers/CPEs should also use the ADSL at the highsite as the primary link and the one on the left of the image as the failover?

Yes, that is exactly what i require.

How would i accomplish this?

If this is not possible/too complicated then i just need to be able to set it up so that it uses the ADSL on the left of the image as the primary (and only) link
Reason for this is that the ADSL on the highsite is currently faulty.

Its basically a matter of routing, and whether or not you are using NAT, and do both ADSL routers go to the same place, (and do you control that place).

Assuming the DSL routers each have their own public IP out to the internet, and you're using NAT, and not using BGP/OSPF, you need to change/add your 0.0.0.0 default route in the appropriate places, and you may have to move the NAT rules to the CPE router. Basically it all depends on what your setup is currently.

Automated/transparent failover is easier if you are using routing protocols (OSPF/BGP, or possibly MPLS), and also if you are not doing NAT. Otherwise, you may be able to do something with multiple 0.0.0.0 gateways with different metrics.

Otherwise, you may be able to do something with multiple 0.0.0.0 gateways with different metrics.

marklodge; that would be the first place to start before going too advanced.