Water going hot then running cold - would magnaclean help?

First you need contact your local heating engineer (Gas Safe) he will investigate the cause, if the cause is sludge/debris in the secondary heat exchanger he will either replace with new or clean the existing. If you live in a hard water area and you do not have a inline lime scale reducer, he will also recommend that one is fitted too. And lastly as you have suggested a magnetic filter fitted to help prevent a reoccurrence of this situation. But depending on the cleanliness of your heating system a power flush may be also be required in more serious cases.