Lombardi et al (1997) found very high germination rates of T latifolia at a fluctuating 20-30C, with plenty of light. They suggest each cat-tail contains 1000s of seeds. If the anticipated invasion of the pond doesn't materialize next year, in the October I'll harvest a few and then sow their seeds in the poly tunnel the following spring. A few dozen plants should be enough. Eventually, I want the whole pond to be a solid mass of 8ft tall bulrushes.
Lombardi, T., Fochetti, T., Bertacchi, A. and Onnis, A., 1997. Germination requirements in a population of Typha latifolia. Aquatic Botany, 56(1), pp.1-10.