Are you looking for a particular place in Canterbury, or just the average / downtown altitude?
Can you give me the particular street intersection you are interested in, to narrow it down to the exact location? If not, I'll still have to give you a small range.
Using Google Earth (http://earth.google.com), I was able to determine the following:
The altitude where Barton Mill Road hits A28 (Sturry Road) is 13 meters.
The altitude at the very end of Barton mill Road is 8 meters.
The slope looks pretty steady, so if you are part way down the road, then it's about 10 or 11 meters.
Let me know if you have any questions, and if you are all set, thanks for accepting,