my  dog who has  successfully  sired several litters before went to mate a bitch several days ago and prematurely ej*culated   he then was successful 30 mins later but would there be live sperm there to reach eggs?

Lap of the Gods!!!   I think, much as obviously you want to get her mated sooner rather than later, I'd have tried again at the end of the day rather than 30 minutes later.   I'm not qualified to know whether he'd have had any viable sperm left to be successful 30 minutes later.   Perhaps others do?

Might not be any help but I was watching a Scottish farming programme a few weeks ago when the AI man took 4 samples in a row from a Bull, if memory serves they were all about the same size sample wise. 200 straws each at £25 a pop

For dogs I'd guess yes but not as many second time round.  Was it the only mating ? Can you not try again today ? Guess its a case of wait and see.  Plenty of tales on here of slip matings resulting in big litters.

Good Luck

Are you sure it wasn't pre-ejaculate?  At our recent mating, the stud dog slipped out and clear stuff came out of him - but the stud owner knew he hadn't ejaculated yet, so they went again and we got a tie. 

Ejaculate is white/cream, pre-ejaculate is clear...