You just need to trim off the dead flower heads. This will keep them lush and bushy, and you can shape them by doing this. If need be, you can also prune them heavily if you need to regenerate them. Basically the bushier they are, the more flowers you will get. Remember that they naturally occur near creeks, so you will have to keep the water up to them in dry weather (although they can still survive long drought conditions).

Trimming off the dead flowers can also give you a really good second flush of flowers in summer.
It is surprising, for a plant that does, as pos says, occur naturally near creeks, that it copes so extraordinarily well with drought. They are used as street trees, well away from any creeks, and get no watering at all once they're past their infancy. They continue to thrive in very difficult drought conditions indeed. So if you're trying to save water, don't waste any of it on this plant (except maybe after a heavy pruning.)
Trish

Thank you both for your replies. There are no flowers on them now of course but I think the knobbly seeds on the ends of the spikes are old flowers? Shall I cut below those? Also, next year shall I keep cutting the dead flowers as they die down or do it end of May once the main flush is over? Thanks for all you help...much appreciated.

As the Callistemon bloom on new growth in any case, you can pretty much prune them at anytime of year to control size. It probably does make the most sense to do your major prune after the initial spring flush of bloom, and limit any further pruning beyond early fall to just light shaping for the errant branch, to get maximum flowering the following spring. Ultimate shaping of Callistemon viminalis is dependant on whether you want to keep them at a certain height indefinitely, or whether you will allow them to become tree sized and ultimately remove lower branches to make them more tree-like. Both approaches are done here in California, where they are more often sold as small single trunked staked trees for use as smaller sized flowering street trees.

I have found it very difficult to keep large shrubs going in large planters over the drought we are currently experiencing across southern Australia.

The combination of heat and transpiration very quickly dries out the soil and the plant can suddenly droop from water stress even though it is a very drought tolerant species.

Part of the drought tolerance of many native shrubs and tree comes from their deep root systems that can reach water sources deep in the soil profile. Obviously this is impossible in a planter and so highly drought tolerant species can be susceptible to fatal water stress.

I would suggest cutting off the bottom of your planters and allow the root system to grow into the sub-soil while maintaining the illusion of the planters.

And please be aware that Australian native plants can be highly invasive environmental weeds when grown outside Australia.