I recently bought a second hand Fiat Doblo and only got 1 key.

After visiting a fiat dealers to get one cut they wanted around £100 for a replacement.

Can anyone reccomend a less costley way of getting a replacement key for my 05 plate Doblo?

£100 for a remote key is a good deal. i'd take that option because sourcing the key elsewhere still leaves you in the position of paying a dealer to do the programming.

Shop around local dealers.
Also some independent auto locksmiths can supply and program normal and plip keys.