Probably just a very snug fit. Lots of 'wiggle wiggle' will probably eventually separate body from chassis. I have had a couple of Britannia loco bodies like this, and you just have to close your ears to the ominous creaking and cracking noises and keep going until separation is achieved; then you can be amazed that despite the force applied there is no visible damage! Since the decoder socket is in the tender, the manufacturer expects the tender top to be removed to gain access; if you don't want to risk doing it yourself contact Hornby customer service. They have a good record for sorting out such problems.