jacob wrote:

Thanks for that. Infact after hours of concentrated thought and a
couple of stiff drinks I came up with the same answer! I thought
perhaps 3 pieces about 18mm square. When removed the mortar would
slump a bit but hopefully it would be even all round. If not then tuff
titty it'll have to stay there - if it doesn't drain properly I'll
just have to flannel it out.

Yes those sound the same size battens I used. The mortar mix wants to
be really pretty dry, so that should stop any slump. The mortar bed
should be level anyway (courtesy of the pre-prepared, level baseboard
you prepared earlier!) before you lay down the shower; when you slide
out the battens afterwards you just want to wiggle the tray ever so
slightly to and fro, left and right to ensure it's properly bedded in.

David