In reply to Si dH:
A set of rods is always worth having - not least of all because drains have the same sensor as photocopiers/printers, which tells them when is the most inconvenient time to fail (like our drains at 5pm on Christmas eve)
Find out where the blockage is - your land - probably your problem. Shared drain - probably someone else's problem.
If you can't shift it mechanically with rods, you'll need to call someone out. In my experience, the drain cleaner stuff only works for the small bore stuff inside the house.
For context, before shared drains became a utility company problem, we had a blocked drain in the road and we were the first house uphill from the blockage. Not great - ended up having to get the road dug up. Because there were more than 10 houses affected, the insurers wouldn't get involved and left it to the local authority to fix as a public health issue. That was going to take six months, so we did it ourselves and got the money back in the end.
Our insurance company was spending £500 every 5-7 days on pumping out the drains so that they didn't flood the garden, but the cost of that was less than digging the road up. Nothing quite like lifting a manhole cover twice a day to check the levels (we had 5 feet to play with), but you do get used to the smell....