Full fitting instructions will be supplied with replacement elements.
Please ensure all servicing (including the fitting of replacement elements) is carried out by a qualified electrician.
see here for example: https://www.dualit.com/support/2-sandwich/spares
Elsewhere in the FAQs, they use both a lighter and stronger wording in the same document (emphasis mine):
Spare timers are available from www.dualit.com and must be fitted by a qualified electrical engineer.
Fitting a new timer is a simple procedure but should only be done by a competent electrician
Meanwhile, for their classic toasters, manufactured in the UK, they supply key spare parts and repair information (when you buy the part) - which is better in terms of Right to Repair, than some of their competitors.
Have you come across other similar messaging by other companies? I wonder whether we will see more of this if manufacturers are required to make more parts available in the future, and whether this wording is considered a “safe” trade-off by manufacturers in reducing their liabilities for repairers not performed by their engineers or another professional.