Jac, you said the robins and great tits are too far away. Is one of the problems the reach of your longest lens?
If so, I can understand that. My longest lens is 300mm and for my smallish back garden that is not enough for me to get a decent shot of a small bird, easily.
To get closer you need to get the birds to come to you, which could be some form of camourflage. This could be a hide but also setting up a feeder close to an open window but draping some fabric around the opening to disguise you and the camera.
For some birds an alternative is different food. Robins, which are very tolerant of humans anyway, will do almost anything to get live meal worms. It takes a bit of time but getting a robin within a few feet is fairly easy. Great tits are also fairly tolerant if live meal worms are on offer.
There is also an element of knowing your subject. Finches tend to sit and eat (and also spill loads of seeds on the ground), so are a little easier to photograph. The tit family do not sit, they grab and go, so catching them is more difficult.
I find sunflower hearts the best seed food; all the garden birds appear to like them. In my garden peanuts get ignored as do fat balls, but the square fat cakes are well liked, especially by starlings.
Dave