damaged bed

i bought a kids bed from Bedzrus, it arrived in 3 heavy boxes which were left in the hall of our house. They arrived on the Thurs. On the Sunday I opened the boxes and carried each piece up the stairs one by one (i am a female). I assembled the bed, could not find instructions but it was fairly simple. When all completely I noticed a small 1" damage to top edge. Not a massive problem but I thought I would contact the seller as over time it would probably fall off. Sent email that Sunday, and received a replay that evening from the MD, he was quoting the trading law about receipt of goods etc, but then contradicted himself and said unless I provided photographic evidence he would not be able to provide a replacement panel. I emailed him photos back on the Monday, I would have sent emails with original email but it was a contact form so couldn't. Never heard from him, sent a reminder the following week, still nothing then sent him a 3rd reminder this time sarcastic, and that was on the Friday nearly 2 weeks after email sent. Received a reply on the Saturday to say that as I had assembled it I took liability of the item. Is this correct?
Also in their email was a paragraph saying that 'surely I would have noticed the damage when I took it out of the box'. That irritated me, as if i had noticed it then I wouldn't have assembled the item. You have to remember here that it wasn't practical to dismantle the bed and then reassemble to old cot and then have to store the new bed somewhere.
Where do I stand? Am I entitled to a replacement part? Their tone is really winding me up, and I have since googled them and found loads of negative reviews of their after care and customer service, Thank you!!

thanks Tony, so even though I assembled the bed and did not check the bed for damage I am entitled to a replacement part? They are having me believe the contrary and are hinting that I might have damaged the part myself? Thanks

thanks tony, i spoke to Trading Standards and lodged a complaint and they advised i use their template letter and send recorded tothe company. i received a recent reply from the company reitterating that i had accepted liability for the item since i opened and assembled the item and i would have to pay for a replacement part, it sent backa one liner saying rubbish and that i had taken legal advice and they would be receiving a letter shortly. strangely they replied asking for all correspondance to be done by email so they could forward onto their legal representatives. they are a very very strange company, as surely they know that emailing letters is not a very secure thing to do etc etc. they come across as bullies, i have also written a review on one of the consumer websites. i doubt they will resolve the matter, but it is a matter of principal rather than the damage to the bed which is only small.