Lancaster had held a 9-7 lead at the end of the third quarter but then goals by John Sissons, Chris Taylor and Stuart Fennell put Bristol 10-9 up. The advantage didn't last long as Alex Parsonage and Granahm Harrington scored for Lancaster - only for Taylor to level it with little over one minute to go.
The key Bristol exclusion happened with 20 seconds left on the clock - and Ellwood made no mistake. Lancaster therefore retain a 100 per cent record after playing nine Division One and Super 5s games this season.
Earlier, Manchester suffered their second defeat of the weekend - 9-4 against Cheltenham. Cheltenham were never pressured after leading 2-0 after the first quarter and 4-1 at half-time. David Taylor scored three of the Cheltenham goals and Rob Parker two.