I located several jars of our home made tomato chutney from a year ago. It was a thin batch tho the taste is ok. Can we tip all contents back into a pan and either reduce or thicken with flour , and refill the jars for further months of storage? What’s best to do ?
There is no general safety rule against reusing canned food in a new batch of food-to-be canned. For example, I have seen recipes for home-canned tomato sauce based on tomatoes bought in a commercial jar. So, that would not be a limiting factor.
What is a limiting factor is 1) the safety of the original canning process, and 2) the suitability of the new product for canning. The first one is simple: if the first batch was unsafe, for whatever reason, you can't turn it back into a safe one.
The second is that you have to use a recipe that is safe for canning. For example, you cannot thicken it with flour, as flour- and starch-containing recipes are not safe for canning. Since you will not be using a known-safe recipe, but "inventing" your own (e.g. by reducing the older one) you will have to measure the pH with a reliable method (not strips) to make sure it is acidic enough for water-bath canning.