Hi James and welcome to GoY. Where are you in the UK - like all the cane fruits Tayberries need a lot of sun to properly ripen the fruit, slightly under-ripe fruit will be sour just as a blackberry is.

all i could say is leave the on the bush as long as possible,

i dont think you can do anything to the soil to help.

make jam ?

I suspect they are being picked before fully ripe. As MG says the amount of sun also has an effect. lots of sugar and cream would be my suggestion .[mmmm.]

I would like to thank seaburn girl moongrower and eamonn-stan for their advice

You are welcome James