Is this killer a fungus

It looks like roots. Have you got some trees or large shrubs around?

There is a sycamore behind the area and a conifer, holly and chestnut tree to the side, but this matting carpet effect extends about forty feet down the side of the garden. The photograph is shot with macro so is magnified, the 'roots' are very fine

i agree with spritz, it def looks like roots

You can cut them off without killing the tree/shrub they are coming from. Then, although it's hard work, you could dig and add lots of compost to enable you to plant the area up. I have done this in 'rooty' areas in my garden. Every time you hit more, use your secateurs and chop. I know it's a daunting task, but do it in small setions and you will win in the end!

Thanks to you both. I've dug and added compost and bone meal for ten years now but I'll keep digging but now with more optimism. I'll also see if I can uncover a better photograph of the juicy red ones on my travels through the bed on the off chance that there is a more sinister explanation. It's a soul destroying, not to mention expensive bed. Dryopteris felix mas seems to be the only thing that survives for any length of time

These certainly are roots , identical to roots on most conifers- recall the roots on your christmas trees?Think they re literally starving your plants of any moisture plants that tolerate dry shady conditions such as your dryopteris , will thrive.Quite a list to choose from so given plants that dont mind these conditions is the answer.Euphorbia , foxgloves , lily of the valley, crocosmia , sea holly ,geranium ...