the main risk is the same as the main risk from the baby jabs- from the pertussis part. (although see fio's thread about her ds's leg swelling up- that's perhaps a more common problem than something nasty from pertussis). By main risk I mean really nasty reaction that you don't fully recover from- very rare though, but its the pertussis part that is the most likely to do that.
How was your dd with the baby ones?
As for the things they protect against:
Diptheria not present in UK
tetanus- well yep protection that's worth having
pertussis- all sorts of issues with pertussis - no time to go into it now, but the disease itself shouldn't be a particular problem to a 5 year old - although can be unpleasent and very nasty if spread to a baby of course.
Polio not present in UK
Weird that they don't boost hib or men C. - well men C I can understand if they boosted it at say age 14 or something, but weird about hib (but bet that's because over 5 year olds only caught it reallr rarely in the past as the majority of people had developed natural immunity by 5, so presumably it hasn't been taken on board that the age profile may change if you vax babies against it).