I replaced my Potterton EP2002 myself about 10 years ago. Both sliders have now broken, and I've bought another unit off Ebay. I remember you open the front panel by pressing in the hole under the bottom right hand side, and I've seen this confirmed by googling it. However, it's not budging and I'm afraid of breaking it! I can't remember it being this hard before. How hard do I have to press? Any tips? Thanks!