Re L G Thoms – any records will still be with the MoD and would normally only be released to Next of Kin, however, if you are not NoK, as he is deceased (quote CWGC entry) you can obtain some information via a Freedom of Information application. There is a charge and it can take over 6 mths to receive any record, see http://www.veterans-uk.info/service_records/service_records....
for details and forms.
Re S Dunne
I understand that the Sebundy Sappers & Miners, that were originally part of the army of Bengal Presidency under the EIC, came under British Crown control in 1858 following the Indian Mutiny and was disbanded in 1863.
Any records will be held in the records of the India Office collection at the British Library in London, but are not online.