They flower on shoots off older stems so not too surprised really. Train in these new stems to form the permanent framework. Next year, many of the buds along their length will produce shoots that should have flowers. In mid winter that year, prune back these stems to a few buds at the base of each leaving each year 3-4cm of that years growth. Any further back into older wood and you will just promote shoots and no flowers.

Don't prune in spring either or it will bleed to death as the sap rises.

A lot of climbers, grape vines included, and more famously perhaps, wisteria, spend their first few years growing, not needing or feeling as though they have to do anything like further themselves by producing seed. Only when they're mature do they start to produce flowering and fruiting wood. As Fractal says, it'll start next year probably. Phil J

Just a thought, is it actually planted in the greenhouse?

Having read about your pond Homebird I wondered about your newly planted grapevine. Heron asked if it was planted in the greenhouse but didn't follow up on it. A vigorous grape can quickly overwhelm a greenhouse. Even outside, which our Pinot Noire grows, we chopped 14 feet off the end of its 40 foot length. It has been very 'active' this year in response!!!! The usual thing with a greenhouse grape is to have the root outside. Take it back right to the main stem in late autumn when established. The flowers are produced very early on our vine, so bees can pollinate, as the leaves get so dense. I will post some pictures of the grapes when the sun gets on them later.