The reality is that if you have it over insured and in the event of a claim you will only get the cost of replacement less 250/500 euro or whatever sum you have agreed in your policy. On the other hand if you are under insured you will be cut by the proportion that you are under insured by. The banks rarely if ever check what you are insured for unless you cancel your insurance altogether. I cannot see how they could enforce that clause because there could be a situation in the future where you may have to increase it to protect all interests.