As above post. Get yourself either a bolster and lump hammer (very slow going lol) or get down to screwfix and get the titan drill for £60 ( http://www.screwfix.com/p/titan-ttb278sds-5kg-sds-plus-hammer-drill-230-240v/97533#_=pas ) it can be used as a breaker with the roto stop feature. chisel...
Actually I can
You are giving the impression that it should only cost a grand and the op is getting shafted
If that's all your porch cost you then well done and congratulations but at least post pics up of it so the op can decide if his could cost around the same or not if he was to go the DIY...
Self levelling will not work. You need to do a search on U Tube and see how to get a good finish on Concrete . Its not hard just time consuming and mind numbingly boring
If its only 20mm a good sharp plane would easily do that job and by the time you've shaped and sandpapered it your only talking 30mins It would probably take you longer than that to rig up the dust sheets set up the router and a possible jig then do the work and clean the mess up.