Chords used:
E5
B5
A5
G#5 


This is like my FIRST EVER
TAB so understand if it sucks completely lol.
and don't be too harsh!

e|----------------||
B|----------------||
G|99--------------||
D|99-999-77-6-7-9-||
A|77-999-77-6-7-9-||
E|---777-55-4-5-7-||

thats it for almost the whole song, (or is it? :0 for all you know i may have done it 
wrong! well it works for me anyway) after that there's another part when it goes 'Na na na 
na' but i haven't watched Horrid Henry for ages so I'll have a look and listen to see what it is.

ok so i listened to it again and can add to it xD
             A5               D5 
         e|---------------------||
         B|---------------------||
         G|-------------------7-||
         D|--7----------------7-||
         A|--7----------------5-||
         E|--5------------------||
so when they send you to your room

e|------------------||
B|------------------||
G|----------------9-||
D|--9-------------9-||
A|--9-------------7-||
E|--7---------------||
    sing a little tune
  

erm..ill come up with the rest later             
thank you :)