Put it on your phone
There is nothing to download and no app store. This puts a shortcut on your home screen that opens straight to your drains, without the browser bars in the way.
Something else
Some browsers cannot add a site to the home screen at all, and yours may be one of them. Nothing is lost: the site works perfectly well in a browser, and a bookmark gets you back to it.
Using something else?
Android, in Chrome or Edge
Open the browser menu, the three dots at the top right, and choose Install app. If you do not see that, choose Add to Home screen instead. Both do the same thing.
iPhone or iPad, in Safari
Tap the Share button, the square with an arrow coming out of it. Scroll down the list and tap Add to Home Screen, then tap Add.
This only works in Safari. If you are reading this in Chrome, Firefox or another browser on an iPhone, open adoptadrain.org.uk in Safari first, then follow the steps above.
Desktop, in Chrome or Edge
Look at the right-hand end of the address bar for the install icon, a small screen with an arrow pointing into it, and click it. If it is not there, open the browser menu and choose Install or Apps, then Install this site as an app.