Does anyone remember this picture book (prob 80s)?

[Oobalooba]

I have vague memories of a picture book I used to read as a child, it was in the 80s.
Really vague!
Certain images I can recall are of a girl going underground, maybe through some kind of trap door?
I think jam is involved somehow?
And maybe knights and / or a dragon?
I've been searching on and off for years with no luck!
Is there anyone that knows what this book might be?

Thank you!

So i have come up with 2 possibilities

The dark hills divide or the trapdoor mysteries

Or

The Secret World of Og

The OP said ‘picture book’ so quite clearly a book for a child much younger than the ones suggested.

Yes a picture book for younger children. I don't remember it being dark or gothic. I was really hoping it would trigger someone else's memory!

It wasn't one set in a school where the kids were learning about Persephone, was it? And then the teacher got married?

The Tunnel by Anthony Browne?

Is it 'the princess and the dragon'?
Pic attached might take a while to appear

I don’t think there’s jam so it may not be right, but John Burningham “time to get out of the bath, Shirley” has a girl imagining she sails down the plug hole out onto a river and is caught up in a fairytale make believe world with knights etc all while daydreaming in the bath.

Ok. No need to be narky... Is it the heat?

All good suggestions, thank you! I don't think any of them are it though. Hoping I didn't just dream it! Really appreciate the suggestions

Could it be ’On the Way Home’ by Jill Murphy?

Could it be a picture book version of Alice in Wonderland/Through the looking glass?
That has girl, tunnels, jam and knights, but no dragon

Shirley Hughes “Stories by Firelight” has a weird underground dream sequence. With little creatures, but not dragons. Can’t remember there being jam.

Might it have been Ogden Nash's Tale of Custard the Dragon?