I have written a proposal for funding which was not successful. However, now that it is written, is it a good idea to send it to a number of other funders?

Why was the proposal unsuccessful? If possible try and get feedback from the funder to help you with future proposal development. It is a good idea to step back and review the proposal again to see where it might be strengthened. Every funder has different criteria and different development areas they are particularly interested in. It would be best to review your proposal again and adapt it according to specific criteria set by the grant-maker, so that each approach is tailored. This would be more effective than sending out exactly the same proposal to a number of funders at the same time.