I am battling with the urban maps. I've tried using infantry supported by heavy infantry to take on the orks in buildings and battlewagons etc by advancing under cover. This means I have to break the infantry into small groups as they advance. The Rokkit and Kannon Speedstas wipe the floor with me because the small groups are too weak. My artillery can take out some of the Speedstas but even one is lethal.
I've tried using Chimeras and Leman-Russ supported by the infantry (line abreast for more firepower) but they are blown apart quickly. So far Shadow Swords or Titans supported by infantry work best for me, but they cannot advance if the streets are too small.
Are there better tactics out there?

Kinda late but try air strikes and thunderhawks with space marines.

Thanks for the response anyway :) In the end I beefed up my artillery to take out the most troublesome units before they came into range. Used superheavies where I could (tanks or scout titans to draw fire - the scout titans were great for getting Ork units to reveal themselves). And kept using plenty of heavy infantry to take on Ork units from range, with Planet Guard to draw fire and then rush the objectives when the Orks were almost wiped out.
Using Thunderhawks can be a good strategy but you need to be lucky to avoid losing them to Ork fire. Can be useful for flanking.
I never found out how to call airstrikes as the Marauders were always greyed out even when I had "All Imperial" unit options.