You will be wasting your time carrying out a chemical DPC injection. There are better ways of remedying damp to a property without injecting a DPC. You may also cause a longer term problem elsewhere within the property. This is a little understood issue about damp. Buildings that have stood for 100's of years in some cases do not suddenly develope a need for a DPC. If you seek advice from a specialist contractor, they will recommend one as it is their job. You would be better off seeking advice from a Chartered Building Surveyor specilising in damp remedial works. His / her advice will be totally independent and will likely involve only basic building measures.
Chemical DPCs can cause damp problems to manifest elsewhere in a proeprty by preventing natural moisutre vapour flow that occurs, typically with older properties. I could go on for hours on this subject, suffice to say, please dont waste your time and money. Do it properly first time and you will not need a guarantee that only lasts for 20 years as you'd have cured the problem forever.