A damaged suspension is a common cause of failure in a 400 day clock. Unless there is some sort of transit locking device fitted to secure the pendulum (and these are only seen on later clocks), these fragile suspensions will kink or even snap if you lift or try to move the clock with the pendulum attached so always unhook it first.
Horolovar in the US is the biggest producer of suspension springs in the world. They use an Invar alloy made from iron (48%) nickel (42%) and chromium (5%) called "Ni-Span C 902 " that has special thermo elastic coefficient qualities to withstand expansion and contraction and this improves time keeping. You can buy a pack of three suspension springs from any decent material house but the problem is identification and you must use the correct thickness for your clock or time-keeping will be poor. They are all about 7 inches long so you'll need to cut the suspension to the exact length needed so that the pendulum does not hang too high but does not come into contact with the base either.
There are four common difficulties to overcome if you haven't fitted one before.
First, the suspensions are as thin as a human hair but come in a range of strengths. Only the right one will give acceptable time-keeping, so you need to know which one suits your clock. Horolovar produce over 20 different thicknesses (Image 1) but don't assume you can just measure the existing suspension with a micrometer because if it's just plain steel or bronze, it won't match the properties of the Horolovar replacement and the clock will run much too fast or slow and well outside the scope of the pendulum adjuster.
You can generally determine which is the correct size suspension by reference to Horolovar's own 400 day clock repair manual (Image 2) but this isn't always the answer as your clock might not have the original pendulum fitted.
However, if you've owned it from new and are sure it has the original pendulum, then email me a good photo of the back of the clock movement (Image 3) and I'll post you the recommended suspension spring for £10 (or two for £15).
Second, you'll have to remove the top block, the bottom block and the fork from the broken suspension and fit them to the new one. To get them in the correct position, you'll need the old suspension as a template, plus a good watchmaker's screwdriver for the tiny screws and a pair of parallel closing pliers to hold the parts securely (Image 4). Use ordinary pliers and the parts will fly off the table into oblivion.
Alternatively, if you have lost the old suspension or any of the fitting attached to it, and your clock is a popular one, I might be able to supply you with one ready made to fit for £30 to £35.
Third, once fitted, you will have to set the clock in beat if it is to keep going. To do that you'll need a beat-setting tool and probably a beat amplifier so that you can clearly hear when the teeth of the escape wheel drop onto the left and right pallets. It's also helpful to have a protractor template (Image 5) to measure and equalise the amplitude each way. Trust me, setting these clocks in beat is an art that only good clockmakers possess so you'll need a lot of patience and a good understanding of how the pendulum is driven by impulses on the pallets as the escape wheel turns.
Finally, you'll then need to adjust the pendulum for timekeeping. This is the easy part so long as you don't have to thin the suspension spring by stroking it with emery if it runs too fast, of course.
By now, you'll have realised why your local clock repairer showed little interest in fixing your 400 day clock (or why they estimated the cost so high); they might not have the skill set, the patience or the specialist tools so have to send it away to someone who has. And given the time this all takes compared to the market value of these under-rated clocks, there's not much room for profit.
If you bring me (or post me) your clock, I'll fit the correct suspension and set it up for you as part of a routine service. Please, though, don't send me any glass domes!