I would really recommend offering a better rate for the work - most artists expect between ÂŁ10 and ÂŁ30 per page so offering just ÂŁ50 for a whole stack of pages is asking for a lot.
A page can take 2-3 hours to create. That means 18 pages easily takes 36+ hours. Would you work a job that paid you for a full week's work with only ÂŁ50? That's paying less than ÂŁ2 an hour.
Obviously you're not meaning to put people off or make them feel put out, but I'm just trying to give you a bit of perspective. Some of us do commissions to pay the bills so we would want twice the amount you're offering at the very least.