If you're thinking of complaining to 'head office' make sure you know which company to write to. Many of Tesco's cafes aren't run by Tesco (even though you still get Clubcard points on purchases). In general, it's only the smaller Tesco stores that have cafes run by Tesco. All of the Tesco Extra stores and most of the larger Tesco outlets now have cafes which are run, under franchise, by Eurest (which, incidentally, is the same company which runs all of Sainsbury's restaurants).
My experience is that the Eurest-run Tesco cafes are far better than those which are still directly under Tesco's control but it's possible that you may have experienced an exception to the rule. Look for a sign, usually by the tills, saying that the cafe is operated by Eurest. (Alternatively, check when breakfast is served. If it's all day, the cafe is run by Tesco. If it's only up until 1130, it's run by Eurest).
If you need to contact Eurest, the address is as follows:
Eurest
Rivermead
Oxford Road
Denham
Uxbridge
UB9 4BF
Tel: 01895 554554
Anyway, however bad it was, it will still have been better than the vast majority of Asda cafes!
Chris