You are going well over the top. Their website reported no problems, which is not unusual for websites as they are rarely kept up-to-date but you spoke to a supervisor and was told that there was a problem. This cost you 10p and for that amount of money coupled with a small amount of time you are grossly over-reacting. It is not Tiscali's fault that there are problems and their service was far from appalling.
Having no internet connection for 5 hours is not exactly a serious problem, it comes below 'will it rain in 6 weeks time at 1230hrs'. There are much more important things in life. I fail to see why you would want to make a 'formal complaint', there is nothing you or the supervisor could do and it is all in the hands of the repair crew.
You need to calm down and just accept that these things will happen, you were told by phone and you have not missed anything...you could have always read a book in the 5 hours.