Is it possible to replace the Perspex covers to the rear lamp clusters?

I have been told that it is only possible to replace the whole cluster assembly at enormous cost?

Help please.

 

By "Perspex cover" you mean the clear outer layer on the taillights? If so, the taillights are only sold as complete units so yes, you would have to replace the entire unit the cover is broken/damaged. You can typically find a used set for ~$250. If you want to take the cover off, there are DIYs to take the covers off so you can access the insides and modify them. This typically involves heating in an oven or with a heat gun and/or cutting with a dremel.

What exactly are you trying to accomplish?