Thank so much for your replies.
[Existing setup] Yes, I can play stuff through my drum computer & hear it mixed with the drums I'm playing and it will also add a metronome beat if I want which is great for some stuff but it falls a long way short of what I'm looking for. Being able to see where my timing goes pear shaped or where I'm drifting early/late in real time is spectacular in the Melodics app' but I can't put my own tunes in it / edit anything / see music in the format I want or even be certain that the order drums appear in the rows will be the same between tunes etc. so it's a lovely practice toy but frustrating that even with multiple people asking for it, they won't tweak it to let you do these fairly simple & desirable things. So, I'm hoping to find something that will do the job properly because, for me, it would then be a really valuable tool for not just practice but experimentation, composition and beyond.
Drum Groove Trainer looks like it's no longer maintained, wasn't written for my slightly later computer system etc. and those download links always make me nervous after finding some installed more than I bargained for in the past, (stuff that re-directs my browser and is in generally a total pain to get rid of). So, without seeing more reports of it I think I will keep looking for now but, definitely, thank you for the suggestion. I may well go back to look at it.
Using Cakewalk as 57Gregy suggests sounds closer to what I was hoping for although it also sounds like it might end up not really doing the job that well. In a way that's good to know as I don't want to burn days/weeks fiddling around sifting the nearly 2000 page manual (for Cakewalk) to only end up with a mediocre solution that I abandon.
If anyone knows of something that'll do the trick, please do let me know 🙂