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Not sure if the pistons can hit the valves, but the inlet and exhaust valves can hit each other ... i'm sure someone with more knowledge will say for sure though.
What was the engine speed when it let go?
It is impossible to say what damage may have been caused if any. The pistons may not have hit the valves but as LB4 says, some of the valves may be bent by exhausts hitting inlets. Where in Devon are you? You could send me an email if you want.