Our last oven got in such a state that despite having an allegedly self cleaning lining, in the end it was easier to get rid of it than clean it !

That,s not quite true but as my husband loves cooking and insists on open roasting absolutely everything whereas I cover stuff up, he has volunteered for the oven cleaning routine . Every few months the newspapers cover the floor, the oven doors and racks are removed and the Lakeland paint on gel makes an appearance. And I retreat from the kitchen for the duration. I have to say he does a pretty remarkable job but for the effort involved I think £40 is not an unreasonable price.
A couple of people in our road use one of these services and their van reappears at odd intervals so people must think it is worth it.
Frankly though I would rather a clean a toilet than an oven any day !