>>> do you think it's worth stressing the unfit for purpose
No, because that's only relevant in the initial period after purchase. It's the 'inherent fault' bit you need to work on. i.e. "I can't possibly see how the timer display could have failed so quickly if it wasn't actually a poor quality part when it was fitted. Can you?".
If the retailer accepts that the part was dodgy at the time he sold the oven to you, he must provide a repair or replacement. (You can state which of those two options you'd prefer but the retailer is at liberty to substitute the alternative one of your preference isn't economically viable).
If he doesn't agree that there was a problem with the oven when he sold it to you, you'll have no right to a repair or replacement unless you can PROVE your point.
BTW, have you tried asking Smeg for their assistance? (Yes, I know that your statutory rights are with the retailer, and not with the manufacturer, but sometimes manufacturers are keen to maintain their good reputation). If you do decide to contact Smeg, DON'T go straight to their service department (who'll probably only be authorised to offer you a repair that you have to pay for). Instead, call their Customer Relations people on 0344 557 9907 to say how disappointed you are with the quality of their product. (Alternatively email
[email protected] but I always think a phone call, where you can use the tone of your voice to express exasperation, is better).