If you prune it all severely you won't get flowers the following year. Mine was pruned severely this spring and is now back to its original height but with no flowers. I don't think you can have it small and bushy but with flowers, but would be happy to be proved wrong. This may help:

https://www.youtube.com/watch?v=_ikLD92cXkY

Thanks Yorklass ours at the front was transplanted from the back (2 left together by the previous owners) and over the past 3 years I have kept it down to around 3 ft and it's that the one that's flowered! I don't want it to grow much higher. Typically I can't remember when I pruned them last time!!!

They can easily get out of hand!

Yes - they can put on a great deal of height in one season, .
Its often a problem when you are trying to keep a naturally tall shrub down to a smaller height. If you really can't remember which height of pruning was successful perhaps you could just experiment as you have two. When mine was reduced from about seven feet this spring it only had one flower, though it usually flowers well. I hope the link is helpful anyway.

Many thanks !

why not cut down a 1/3 of the stems and then the other growth will have flowers and keep doing this on a rolling programme and it wont get to big. Remove the growth evenly from the bush down to close to the base in spring before leaves break. The pruned stems may not flower but the foliage will be lovely.

I generally prune after the berries have finished (I leave them for the birds), generally in the late autumn and mine flowers beautifully. However, if a branch or two is blocking other plants I lop it off anyway despite knowing that that branch won't flower till the following year. It's a very tough tree I think, like all elders, or at least the ones I know.

Thanks Arbuthnot, yes I will prune both in the autumn. We so like the flowers!

If I were trying to keep a Sambucus shortish, but blooming, I would prune it severely immediately (!) after blooming, and then pinch all the tips when the bush has produced 30-40 cm of new growth. It's a short window: delaying either pruning or pinching can prevent flower bud formation in late summer.
If berries are wanted, too, forget about short, but it can be a more moderate height if you prune this way:
The first year, prune the whole bush severely right after bloom, but don't pinch it later. The next winter, prune out half of the canes right to the stump--i.e., if the bush has 20 canes, remove 10 of them, preferably the tallest ones. The remaining canes will bloom and bear fruit, while new canes replace the ones that you removed. From the next spring on, remove the canes that flowered and fruited, and leave the new canes to flower and fruit in their turn. That's similar to Sbg's method, but on a little faster cycle, which should keep it a little shorter.

Tugbrethil - Many thanks for that detailed reply, much appreciated!