I did a bit of research. Are the things in my garden inspection chambers and not manholes? Does that mean I have to add a rodding eye/manhole between the extension and the inspection chamber?
I always thought rodding is done with a flexible metal hose that rotates, clearing the blockage.
If you want to be pedantic, then an Inspection Chamber is a small pit that you work in from above, whereas a Manhole can be large enough for someone to enter and work within, provided the correct Health and Safety precautions are followed.
Most people now clear/clean sewers with High Pressure Water jetting gear, which will go around corners, but the basic rules of drainage installation still apply, and one of those is, bends in a run are only allowed immediately outside of a chamber. Seems to me the Water Company are being quite generous in so far as they're not demanding you divert the entire run around the proposed extension!