Thanks for posting the picture of your flute mendipman! There is a good chance that this is German made, but its not certain. The integral foot with the lower body is a common feature of mass produced German flutes, and yours has that. The embouchure cut on many mass-produced German flutes is a rounded rectangle and fairly large. Yours, however, is oval, which is somewhat non-typical, and in my opinion preferable! The design of the rings on your flute is also a little different to the rings on most mass produced German flutes I’ve seen. Yours look nicer, and the wood looks very nice too. I think your money was well spent.
As for the tuning slide, that should be fixable. I’ve done dozens of these. Generally, the slide has become gummed up with congealed oils, beeswax and verdigrease that have solidified over time. The secret is to heat it up (a lot!) before trying to move it.
If you have a sturdy pair of work gloves and a heat gun you could trying the following procedure. Take off the crown and push out the cork so that you can see all the way through the head tub. Use the heat gun (probably on a medium setting) to blow hot air through the head tube, alternating which direction you blow from (so as not to concentrate the heat too long at a particular end and start to burn the wood) and giving enough time for the metal liner to become hot and for the heat to penetrate.
While you are doing this, wear the work gloves so you don’t burn your hands while holding the flute, and be careful not to burn yourself with the hot air coming out of the embouchure hole. If you do this for a while the metal lining of the head will become hot, and will start to melt whatever is binding the outer tube of the slide to the head liner. At that point you can remove the heat, grab the head tube in one hand and the barrel in the other, and try to twist them relative to each other. If there is any movement, that is a sign that you are going to ultimately succeed. Just twist and pull repeatedly to open and separate the slide.
As you do this the resistance should get progressively easier as the contact surface area reduces. If it starts to get harder again, it is because the tube has started to cool and the material that is gumming it up has solidified again. The solution is to apply heat again and retry. Once you have the two pieces apart, clean them thoroughly with some solvent (but don’t abrade them – you don’t want the slide to get play in it), apply slide grease, and reassemble. Once the slide works you should then be able to determine how the overall tuning of the flute is.
Oh, and one thing to keep a close eye on when you are doing this is the alignment of the head liner in the embouchure hole. Sometime the slide is so gummed up that you have to apply so much heat that it loosens the head liner (which will typically have been glued in place using hide glue or shellac, that loosen under sufficient heat). Then aggressive twisting to open the slide rotates the head liner and you can see misalignment in the embouchure hole. If this happens, remain positive: if you can move it one way, you can move it back again with the same amount of heat and force. Also, remember that the contact surface area between the outer slide and head liner is generally much smaller than the contact surface area between the head liner and the wooden head, so this works in your favor.
The other thing that might happen is that the heat loosens the glue that is holding the rings on at either end. If these rings come off, don’t worry, just pop them back on again at the end (being careful to put them on the right way around, because they are generally slightly tapered).
