in thinking I could do a better seasonal weather forecast than the Met Office

[bellissima]

...with a handful of seaweed and a decent view from the window?

You will recall the 'barbeque summer'. Then at the end of November we got a winter forecast giving a fairly high probability of a 'mild winter'...

But fear not folks, they've just revised this, yesterday, to a greater chance for a cold winter - maybe they finally went outside for a ciggie break? Or came back from their exotic holiday on the proceeds of taxpayer funding for their great forecasts....

I could do better with the old "piece of string" foreacsting method. Hang the string outside the window:

If the string is moving, it's windy.
If the string is wet, it's raining.
If the string is stiff as a board, and has a huge icicle hanging from it, it's a barbeque summer...

I don't pay any attention to the weather forecast as they tend to be wrong. I just look out of the window - so much easier and a lot more reliable.

I think they just ask various farmers what their cows are up to, and go from that.