Hi Emma and welcome to the site.
Oh, that's a shame that your Simnel cake is turning out to be quite a disaster, especially if you have made it previously. I'm assuming that your previous year's cakes have been good and this year it's not playing nice for you.
If it's stuck to the tin that suggests that the heat in your oven has been too high or it has been baking too close to the edges of your oven, and it has unfortunately welded to the sides of your tin, however, if when you are turning it out it is falling to pieces that suggests either you are turning it out too soon, the cake hasn't baked for long enough or there have been insufficient ingredients binding it together. It may look done on the outside, but the inside hasn't had time to catch up.
Did you skewer the cake before you removed it from your oven?
A skewer is a metal, plastic or wooden stick that is used for a variety of things when baking or cooking. A metal skewer is pointed at one end and has a handle or a rounded-off end at the other which enables you to hold it. These are inserted into a baking cake to see if the middle has...
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I would also try using baking paper or a different type of greaseproof paper and also additionally greasing your tin. I use Bake Easy, which is fabulous for getting cakes out of tins without paper, they slide straight out.
I lightly spray Bake Easy in my tins prior to placing in the batter/mix as it ensures the quick release of cakes after baking and prevents them from sticking to the tins. Other greasing agents such as butter or a light coating of vegetable oil may also be used. Please note that I still line...
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My other way of getting cakes out of the tin easier is the wet tea towel method.
In order to free your cake from the tin much easier then do this handy tip when your bakes come out of the oven. Place your wire rack on the side. Wet a tea towel by placing it under the cold tap and then wring it out. Put the damp tea towel on top of your wire rack. Place your...
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When baking Simnel or any other fruit cakes always bake in the centre of the oven, due to the amount of baking time, I always use a water tray to add extra moisture to the bake.
If you are baking a cake for a long time such as fruit cake and wish to add extra moisture then simply add a tray of water to the bottom of your oven and your cake will absorb extra steam whilst baking. Basic Baking and Cake Decorating Hints, Tips and Tools Recipes and Guides Tutorials
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Remember you can always turn the temp down and bake for longer, also remember if you have been used to using a conventional oven and now you are baking in a fan oven, you will need to reduce the temp by about 10-20°C.
The only other options for a cake collapse are insufficient quantities and other things, but I don't want to overload you just yet if the above helps.
If you need further help, please let me know.