Captain François Gailen de Préval

I'm trying to find out some more background info on engineer Captain Préval who was sent out to Portugal prior to Wellesley's landing in August 1808.

Here is what I have so far:

The List of the Officers of His Majesty’s Foreign Corps of 1797 has a Captain Francis Galien de Preval serving as an engineer with Lowenstein’s Fusiliers, with his commission dated as 27 July 1796.⁠1 Given the frequent anglicisation of foreign officer’s names, it is likely that his first name was François, and in a letter from August 1808, in which he describes himself as ‘advanced in age’ and not in best of health, he signs himself F.P. Préval.⁠2 In a later Gazette entry for a brevet promotion from major to lieutenant colonel he is listed as ‘F.P. Preval, late of the Royal Foreign Engineers’.⁠3 Finally, in a letter to Wellesley, Castlereagh refers to him as an ‘officer of Engineers acquainted with the defences of the Tagus’.⁠4 So, from this scant information, it is possible to suppose that he had been an engineer in the French army prior to the revolution, had left France and then joined Lowenstein’s Fusiliers. When that regiment, along with many of the foreign corps, were merged into the 60th Foot and lost their artillery and engineers he carried on in British service, using a shorter form of his name, and at some point served with Stuart’s force in Portugal between 1797 and 1802.

If anyone could confirm his French (or other foreign) service, or perhaps where in Europe he came from that would be great. There does seem to be a French family with Caribbean and then American connections that comes up if you search for Galien de Preval.

1 TNA: WO 65/168: List of the Officers of His Majesty’s Foreign Corps.

2 TNA: WO 55/977: Préval to Morse, 26 August 1808.

3 The London Gazette, 9 July 1814, p.1392.

4 TNA: WO 6/185: Castlereagh to Wellesley, 30 June 1808.