You don't need peat in the ground to plant shrubs, what you do need is good humus rich soil. It you simply removed all the couch grass and planted the shrubs without doing this then you do have a problem. The only solution I can see if to clear away the gravel from around the three shrubs and fork in some compost or soil improver from your garden centre. Also make sure you keep the shrubs well watered so that they have a chance to get their roots settled into the soil.

I think you have stated the problem in your post "I may have bad drainage in this area." Keep in mind that plant roots need to breathe air as well as drink water. I think your plants are drowning and the excess water is causing decay to set in. This will happen with whatever you plant in that area except for bog grass or swamp plants. You need to raise the level of the bed. You can keep the gravel and peat in place. Maybe just frame it with 2x4s and mound it up with new top soil high enough so that your plants aren't sitting in water and the water drains away from the roots. You should see immediate improvements and your plants will be happier. Good Luck

If you do raise the level of the bed you will need to lift the shrubs and replant them so they are still at the same level they are now. Why do you think you have bad drainage?

I am only assuming I have poor drainage because I can't think of any other reason for the plant going brown and miserable.
The ground around that area that I've cleared but not planted with anything looks very thick muddy clayish.
Incidentally - the plant is an Acer.

I thought peat was humus or humus making - what should I use then - just usual compost?

There are several reasons why plants go brown and look unhappy. Acers, assuming you mean the Japanese Acer, need acidic to neutral soil which needs to be moist but well drained, heavy clay is not something they are going to appreciate. Also need to be in a sheltered semi-shady position. I'd use soil improver, particularly as it is now difficult to get peat, BUT even that isn't going to really help if your soil is heavy clay. If they are small enough I would lift the three Acer trees and put into large containers with ericaceous compost and look for something a little more tolerant for the border, or stand them on top of the gravel. Remember to keep them watered.

Please could you recommend what would be more tolerant?
I want things that will grow large and be pretty-ish [verigated]
Also, what is a soil improver?

You could try a red stemmed dogwood for one of them if the site is sunny. Not much to look at in summer but in winter the sun on the red stems is really beautiful. In spring you cut off all the red stems and they sprout new ones from the base (only the new wood is red) Some shrub roses grow quite tall and bushy and some have lovely hips as well as the flowers - and they love clay soils. It would be nice if people in garden centres made sure people buying acers knew what growing conditions they needed - it would save a lot of heartache! For a variegated evergreen have a look at Eleagnus maculata.
Hydrangeas like moist soil - try the white flowered Hydrangea paniculata which has been recommended as tolerating clay.

Soil improver comes in 25 kilo bales from your garden centre and helps to break up heavy clay soil.

Do you know what it consists of MG?

I've just checked with Bulba Stera and we use New Horizon Multipurpose peat free organic compost now. Seems the 'soil improvers' no longer exist! Shows how observant I am... The sales blurb states: A 100% organic and peat-free compost made from recycled renewable resources of UK origin, which gives them a lot of scope in my opinion.