Does anyone know how much roughly builders public liability insurance costs ? Im just doing small roof repairs, chimney stack rebuilds/maintenance , patios, brickwork etc.

Send me a cheque for £100,000.00 you will be insured for upto £50,000.00 instantly, with of course our standard £10,000.00 excess. Thanks to our affilitaion to the NHS we supply a free ambulance if you fall off the roof. Just get someone to dial 999 and say ambulance when prompted.

You might be better looking at package insurance which includes public liability with , for example, employers liability (if you employ anyone) and property damage.
If you find a decent broker they should be able to look at menu or option based cover - you only pay for what you need, the limits and deductibles (excesses) will have an element of choice in them.
Try your local chamber of trade or business link to see if their members have any good local brokers to recommend, maybe your mates/other trades have some ideas - the building trade organisations might have some bespoke schemes as well