My 400d will power up 5 out of 6 times, the sixth time i have to remove the battery then replace it and put a small bit of card on top of the battery push it into place close the battery door and power up. I have looked inside the battery compartment and I see a spring that would take up any slack in the battery but this would only take slack upto the small clip when in the operating position. When the bit of card is deployed it put the battery against the spring. The battery is the original so I have ordered 2 relacement ones but I am not sure that will fix the problem. Any advice would be gladly accepted.
Well, your 400D is well past a fix from Canon. Given that, I would suggest a couple of things after you check with your replacement batteries, but if the dimensions are the same, then likely the issue will remain.
You say: I have to put small bit of card on top of the battery push it into place close the battery door to power up
The spring is supposed to push the battery up and away from the contacts for battery removal, once the latch to secure the battery has been replaced. So, to me, that is unlikely to be the issue. I am more inclined to suggest investigating the orange latch, to see if that has moved out of place. If it has moved up, then it is not locking the battery in the right location.
One way to resolve this is to look for a battery grip (the Canon model for this unit is a BG-E3) - they are still around and you might get one pretty cheap, and there is the benefit of using two batteries plus the extra controls. The battery grip screws into the base of the camera, thus bypassing the catch system completely. These grips are quite small and should not add a lot of weight and bulk to the camera. I have a 3rd party one on my 400D that I got on eBay, and it has worked flawlessly for about 16 years - compatible grips may not be given the same name as Canon's original, so search by the Camera model - 400D or XTi.
Another possible solution would be to get an eraser, cut a slice from it so that it fits easily in the compartment and allows the door to close while engaging the contacts. Experiment with taking thin slices from the eraser- testing it with all of your batteries. If you find that it works consistently, then possibly use a small amount of glue to attach it to the battery door - essentially replacing the function of the orange catch. Make sure that placing it does not foul the catch or any of the closing mechanism.