My online execution-only broker has contacted me to say I have overbought shares and am now financially liable.

I have been dealing very frequently - sometimes over 60 times a day and have relied on the integrity of the company website for account details.
My online dealing page had previously shown that I had shares to sell in several companies which I later sold.
The broker now says the details shown on the dealing page were wrong - that it was a mistake to say I had these shares in my account
- and that the shares have now been bought back from the market - leaving me with a financial deficit.

Am I liable for this deficit - or does the fault lie with the broker for telling me I had shares to sell which I did not have ??

i am no lawyer, but i would hazard a guess that your broker is legally correct, in just the same way that a bank can claim back money that has been accidentally credited to your account ..... i am sure in the fine print of your contract, the broker will have covered himself with an E&OE clause relating to screen info and similar.

However
a broker is dependent upon maintaining a good reputation within the City, so my advice would be to demand to speak to, or even to go and see, at least one of the directors .... do not allow yourself to be fobbed off with a minion, but stay calm and very insistent.

if the broker remains intransigent, i dare say there is nothing to prevent you going public, here or elsewhere, but if you do, make sure the details you publish are 100% accurate or you could be in deep trouble.

If i was your broker and this happened there would be one solution.

You would be right every time and problem sorted no matter what the cost.

"60 trades a day"

Most brokers would bite your hand off to have customers like that.