First off, odds one, many thanks to MSC Boat Products. The ammeter on the 12 volt panel failed and when I contacted MSC they were very helpful and a replacement was provided free of charge when the original unit could not be repaired.
The old Francis type searchlight we have on Yarwood looks really good but it is not an ideal tunnel light as it does not spread much light nor was it designed for a 12 volt system. The only 12v E27 bulb I have found is an incandescent pearl 60w used in an inspection light. I have managed sourced an E27 to GY6.35 adaptor which allows a 50w halogen capsule to be used, with the potential for higher wattage's available. My most recent purchase is an LED which was listed as a 7w 120 SMD unit on ebog. This might not be any more effective than the halogen but for under a tenner including P&P I think its worth a try.
However, as there are no tunnels on the River Great Ouse I will have prance about on the tug deck on a really dark night to do a comparison.