I know of a few phrases that come close - but I'm struggling to think of one that accurately describes in Thai the English (UK) meaning of sad or pathetic. As in "you are so pathetic!" Any ideas?

Much appreciated

น่าเวทนา

I think the closest is น่าสมเพช

I think the closest is น่าสมเพช

I don't think it is the same word.

So pathetic is off the true meaning of the word 'pathetic' : exciting pity or sadness.

I wonder if the poster means the sarcastic use of the word 'So pathetic' as a term of scorn used against a person who is dealing with a problem in a way in which the speaker does not approve.

How about ซมซาน?

Yes, a term of scorn, as often used in UK English

To be clear: I don't mean the true meaning of "pathetic" but it's widely used other meaning in UK English meaning miserably inadequate. Such as: "my earnings are pathetic!" Or "you pathetic (or sad) little person". Is there a phrase in Thai that captures the meaning accurately?

I agree with Mole, น่าสมเพช naa som-pet is closest. Be careful though, a lot of people find being called "miserably inadequate" rather offensive.

To go on with the gist of what the OP is getting at, a pathetic person could easily be refered to as a useless person which would translate as ไร้ค่า

I agree with Mole, น่าสมเพช naa som-pet is closest. Be careful though, a lot of people find being called "miserably inadequate" rather offensive.

Looks good to me, as in this dictionary example:

ฉันไม่มีวันลืมว่าฉันเคยมีชีวิตน่าสมเพชเพียงใด

In American English (no that's not an oxymoron, Brits), "pathetic" used in this way carries a negative connotation. "Sad", might or might not carry such a connotation, depending on tone of voice, facial expression, context, etc. For the above use of pathetic, I agree that น่า สมเพช is best. But, for sad, I think น่าเวทนา is better if one truly means that the situation causes one to feel sad or, in other words, to pity the person being discussed.

In American English (no that's not an oxymoron, Brits), "pathetic" used in this way carries a negative connotation. "Sad", might or might not carry such a connotation, depending on tone of voice, facial expression, context, etc. For the above use of pathetic, I agree that น่า สมเพช is best. But, for sad, I think น่าเวทนา is better if one truly means that the situation causes one to feel sad or, in other words, to pity the person being discussed.

It would seem from Mole and others that the Thai can mean pathetic in the sarcastic sense. But for non English speaking Thais I think that calling someone pathetic to their face to hurt their feelings by using สมเพช or เวทนา might seem odd. แย่ might be closer.