I have a Boch greenstar junior boiler ,leaking a black tarry looking substance.

Get a second opinion, if they say the same thing as the first guy then you know what the answer is or buy a warranty plan from Worcester boilers.

Sounds like an easy fix. Where is the tar coming from. Need more information to answer correctly

If you drop the can drop the front panel down on your Boiler you will see a pressure gauge ranging from 1-3bar, if the pressure is sat below 1 that would indicate the leak is on the Heating side of the boiler, if that guage is still at 1 or above then it's more than likely a direct hot water inlet or outlet. Tarry substance/ colour would probably be a seal on the flow or return valve that's dripping.. but without seeing the boiler & pipework it's pure guesswork.. get a second opinion from another engineer. Good luck

Hello, a 10 year old Worcester that has been regularly maintained like you say could still be good for another five years or more. Therefore a repair is worth it. It's likely a seal or seals which need replacing and possibly a flush of the heating system. Since all parts will still be available for a 10 year old Worcester boiler, it is not correct to state that it 'needs' replacing. Replacement IS an option if a customer wants to do this but so is repairing it. Repairing it is significantly cheaper than replacing it. Also any flush carried out now will still benefit a future installation as the system will need less flushing at a later date. The only time I would ever tell a customer that they 'need' to replace a boiler is if the parts they require to operate it or operate it safely are obsolete and I cannot source them anywhere in the whole country. As the other Engineers have said, get a second opinion from a local Gas safe Engineer, invite one from Mybuilder who has good feedback. All the best.