mortar on concrete

Need to do something about grotty concrete surface outside back door, without lifting it, so wife has decided on decking over it, with a 20ft x 18 inch x 18 inch high flower bed built directly onto the concrete with decorative blocks. Flower bed drainage will consist of holes drilled in the concrete base and drainage weep holes at the base of the wall, with a good layer of rubble etc.
The concrete surface is sound, but somewhat undulating, and so before I start, I think I will need to lay a levelling strip of mortar/concrete the full length of the wall. Any suggestions as to mix, and do I need to prepare the concrete in some way, over and above cleaning it etc, so that the mortar bed sticks?
Is this approach feasible, or am I heading for trouble?
TIA
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Zipadee Doodar wrote:

No, but why bother? If you are laying decking all you need is to ensure that wherever the decking is supported its teh same hoeght as anywhere else. As far as building any struvtire on oit - well my foundations undulate several inches. A level and string sorted that out at first course level.

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