Yorkshire gardens get damp shade, prolonged wet spells and cold snaps, which means the calendar matters more here than it would in a drier climate. The good news is that the cleaning itself is far less weather-dependent than people expect.
Spring
The most popular window, and for good reason: winter growth is at its worst and you want the space usable. Biocide works well as temperatures rise, and there's a decent chance of dry days for re-sanding.
It's also the busiest period, so booking earlier gets you a better slot.
Summer
Best conditions for sealing, because surfaces actually dry. Warm weather also speeds biocide die-back. The trade-off is that summer is when you most want the patio, so the disruption is more noticeable.
Autumn
Underrated. Clearing leaf litter and treating before winter stops the surface spending five months as a growing medium — and it means you don't start spring with a green patio at all. Re-sanding is still achievable in a dry spell.
Winter
Cleaning is still possible, and safety cleans on slippery steps and paths are a genuine winter job. Biocide works more slowly in the cold, and sealing and re-sanding generally have to wait for dry conditions.