Hello Simon,
First of all, Barry is correct in that infantry can only charge Shaken or Disordered infantry or a defended obstacle, where as Tribal infantry can charge anything.It's all there on Page 20 of BLB under infantry charges and melee.
Troops must declare a charge to reinforce a melee and must have room for at least one full base to make contact.Your latest question on your melee situation still needs clarification, but I'll have a go.
Two independant cavalry squadrons charge two infantry battalions standing side by side.The total frontage of the two infantry battalion is 270mm (2 x 135mm) where as the total frontage of two cavalry squadrons is 240mm (2 x 120mm).If one squadron hits a infantry battalion square on then the second squadron hits both infantry battalions as 15mm is free.....I hope your following this......
If the first battalion breaks and routs which was fully hit by one full squadron and part of the other, this will release one squadron to continue pursuit where as the second squadron must continue to melee the second battalion.....Are you still with me???......Follow on.......
If it was the second battalion who broke and routed neither of the two squadrons could pursue as both are in contact with the first battalion and must continue to fight.
The reason why infantry can't charge is that they tended to rely more and more on fire power and the tactic of the period was to march upto a enemy infantry formation and trade drilled volley fire until one of them broke.Pikes were still carried to ward off cavalry, but with the first plug and then socket bayonet these disappeared and cavalry were usually left to deal with enemy cavalry.
Hope this helps................Ray.