Hi wedan69,
I suppose if your going to clean public areas make sure your insurace more than covers the risk involved some local authorities insist on a minimum of £5 million public liability others less. As you have found out yourself some members of the public ignore any warning signs but unfortunately its usually us that bear the brunt of it.
With regards prices, depending what method you use to remove graffiti and from what surface, the price can vary but when I was doing my research into this some people were charging anything from £10 to £ 90 per sq mtr. Total overall area taken into consideration of course but bear in mind all the costs of chemicals, time etc and of course your profit. If the intention is having a regular contract and wish to keep the customer happy then maybe you should offer a discount for subsequent cleans.
Where are you based and who are the LCA? Is it a local authority?
Good Luck
Paul