I don’t bother repairing them any more and just carry a spare tube for the price of them.
is there any reason why I should not buy a £3.25 tube from asda, £
From my experience:
Even the cheapest "Chinese" tubes do the job.
However, Schwalbe tubes seem to hold air best of all - that is, require the least frequent topping up (pumping up).
They also have good quality rubber, so even after 5, or 10 years, they are still OK and can be patched when punctured (Rema Tip-Top is by far the best for this).
Expensive? Yes - around double the price of the Chinese, if not higher. But 3 tubes last around 10 years, or longer. I just put the spare one into the tyre, and patch the punctured one, delegating it to being a spare tube.
A thing I really don't like is when a tube has rubber valve stem. It can get cut on the rim sometimes, and it's more difficult to hold in place when inflating a flat tyre (no steel nut that holds the valve from dropping through the valve hole).