Anyone experienced the same? What's a god alternative? What are your views on the service level ... Just par for the course ?
Resisting any temptation to respond to the typo...
I'm afraid there is not an easy answer to the problem of boiler breakdown. You have a service "Contract". It's completely impossible in "real time", that's during the occurrence of the fault, to make the Contract operate as you would want it, and subsequently is very difficult (as a lay person) to obtain redress for what you see as breach of contract.
The Eon sales page "puffs" that 8/10 are very satisfied, so in making a case for the contrary you are already on the back foot. I have read through the Terms & Conditions and cannot see (exterminator help pls) where response time is "guaranteed". That said *I* would not consider 11 days without heat and hot water to be acceptable.
What to do? Well first of all have regard to 6.1.3 Complaints. I would advise waiting until the repair is complete, then make your complaint to Eon first, before this forum. However the outcome of the complaint would make a really interesting and informative post which I await with interest.
More generally on maintenance contracts, the householder needs to recognise that response times and competence (beyond gas safety competence) is not guaranteed (even if response times are "guaranteed"). This is particularly applicable to the severest weather conditions. It is not expensive to own sufficient electrical heating to make a home "not cold", even warm in selected rooms. With a combi-boiler I recognise running hot-water may be a more difficult contingency. (That may or may not be an argument against combi-boilers). Treat the service call as an opportunity for the service provider to "scr*w-up" but hope for the best. Or vice versa depending on your outlook.
You have a boiler (Viessmann) which is considered to be at the quality end of the spectrum. That makes it relatively uncommon which may explain the apparent lack of familiarity in the service engineer. If I was asked for specific advice (for the benefit of others) I advise the installation of a "middle market" brand (typically but not exclusively or as a recommendation - Worcester Bosch or Vaillant) where the manufacturer maintains its own service organisation and take out the manufacturer's own maintenance contract. That will be the best way to be confident of service engineer product familiarity and the carriage of common spares, which is the key to any boiler repair.
As an aside, I note from the Eon sales page that their cover is cheaper than British Gas's. There just may (or may not) be a message there.