I recently had a triton electric shower fitted to replace an earlier model that had died in my downstairs shower room. Within a short space of time, I noticed when the shower was turned off, immediately, a loud thud could be heard in the pipework upstairs and has continued since. A week or so after this, when the loo is flushed in the same room, a sort of screeching noise can be heard in the upstairs pipes lasting for 4-6 seconds or so. This never happened before new shower was fitted. My wife has recently noticed that if tap is running in basin, before shower is turned off, thud is either reduced or not there at all. Haven't tried this yet when flushing the loo. Any ideas? Many thanks

What you are experiencing is call "water hammer", at least that is what it is called in the U.S. When the flow of water is abruptly stopped by a fast acting valve, such as a toilet fill valve or single leaver faucets (taps) the momentary increase in pressure causes the pipe to move and bang against structural members.

Here, in the past, we used what is called air chambers to combat this. An air chamber is simply a capped, vertical piece of pipe installed above the water line at each fasting valve. Usually 18 to 24". When water enters the system, air is trapped in these air chambers. The air trapped in the air chamber acts as a cushion just as shock shock absorbers do on a car. The only problem is that over time, the air in the air chambers is absorbed by the water.
The corrective procedure is to turn off the water to the house, open all faucets (taps), flush all toilets and drain the system.
When water is restored, the air in the pipes will rise to the air chambers and again cushion the abrupt stopping of the water flow. It helps to leave water off and faucets open for a while so that the any vacuum holding water in the air chambers is broken.

The commercial version of the air chamber is the water hammer arrestor, shown below. These devices have bladder in side that keeps the air and water masses separate, eliminating the necessity of periodically bleeding the system.