Mileage 11700

Sudden loss of power going about 15mph around a corner. Camshaft position sensors both faulted. Suggested timing belt has gone. How likely is the extent of the damage done? 

Assuming the belt has failed in some way, worst case scenario I can think of is the pistons have hit the valves damaging the valves, possibly the cylinder head and pistons. 

Would you suggest a second hand engine replacement or fairly fixable? 

First I'd take the rocker cover off and actually confirm there's no longer a belt wrapped around the camshaft pulleys.

If so then you'll have to strip the head down to see any damage. Sometimes you get lucky and it's just a few bent valves you have to replace at the cost of a few quid each. It's obviously a labour intensive job (costly) if not doing it yourself, but certainly a whole ton cheaper than a new engine. It's impossible to say whether you've done damage to the pistons and require a new engine without someone taking the head off at least.

Can you inform us about your timing belt? It is gone with so few miles?